diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0003/expout b/go/regtest/cases-pending-go-port/verb-merge-fields/0003/expout
deleted file mode 100644
index 15654f5ac..000000000
--- a/go/regtest/cases-pending-go-port/verb-merge-fields/0003/expout
+++ /dev/null
@@ -1,4 +0,0 @@
-a_p0       a_min a_p29      a_max a_p100 a_sum b_p0       b_min b_p29      b_max b_p100 b_sum
-436.000000 436   451.660000 490   490    926   195.000000 195   267.790000 446   446    641
-320.000000 320   379.740000 526   526    846   780.000000 780   833.070000 963   963    1743
-220.000000 220   413.720000 888   888    1108  705.000000 705   741.540000 831   831    1536
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0004/expout b/go/regtest/cases-pending-go-port/verb-merge-fields/0004/expout
deleted file mode 100644
index 3bf54dc47..000000000
--- a/go/regtest/cases-pending-go-port/verb-merge-fields/0004/expout
+++ /dev/null
@@ -1,4 +0,0 @@
-a_in a_out b_in b_out a_p0       a_min a_p29      a_max a_p100 a_sum b_p0       b_min b_p29      b_max b_p100 b_sum
-436  490   446  195   436.000000 436   451.660000 490   490    926   195.000000 195   267.790000 446   446    641
-526  320   963  780   320.000000 320   379.740000 526   526    846   780.000000 780   833.070000 963   963    1743
-220  888   705  831   220.000000 220   413.720000 888   888    1108  705.000000 705   741.540000 831   831    1536
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0008/cmd b/go/regtest/cases-pending-go-port/verb-merge-fields/0008/cmd
deleted file mode 100644
index 43d3c4116..000000000
--- a/go/regtest/cases-pending-go-port/verb-merge-fields/0008/cmd
+++ /dev/null
@@ -1 +0,0 @@
-mlr --oxtab merge-fields -k -a p0,min,p29,max,p100,sum,count -r in_,out_       -o bar regtest/input/merge-fields-abxy.dkvp
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0008/expout b/go/regtest/cases-pending-go-port/verb-merge-fields/0008/expout
deleted file mode 100644
index e18a5ec35..000000000
--- a/go/regtest/cases-pending-go-port/verb-merge-fields/0008/expout
+++ /dev/null
@@ -1,227 +0,0 @@
-a_in_x    1
-a_out_x   2
-b_in_y    4
-b_out_x   8
-bar_p0    1
-bar_min   1
-bar_p29   2
-bar_max   8
-bar_p100  8
-bar_sum   15
-bar_count 4
-
-z         1
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
-
-a_on_x    1
-a_out_x   2
-b_in_y    4
-b_out_x   8
-bar_p0    2
-bar_min   2
-bar_p29   2
-bar_max   8
-bar_p100  8
-bar_sum   14
-bar_count 3
-
-a_in_x    1
-a_oot_x   2
-b_in_y    4
-b_out_x   8
-bar_p0    1
-bar_min   1
-bar_p29   1
-bar_max   8
-bar_p100  8
-bar_sum   13
-bar_count 3
-
-a_in_x    1
-a_out_x   2
-b_on_y    4
-b_out_x   8
-bar_p0    1
-bar_min   1
-bar_p29   1
-bar_max   8
-bar_p100  8
-bar_sum   11
-bar_count 3
-
-a_in_x    1
-a_out_x   2
-b_in_y    4
-b_oot_x   8
-bar_p0    1
-bar_min   1
-bar_p29   1
-bar_max   4
-bar_p100  4
-bar_sum   7
-bar_count 3
-
-z         2
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
-
-a_on_x    1
-a_oot_x   2
-b_in_y    4
-b_out_x   8
-bar_p0    4
-bar_min   4
-bar_p29   4
-bar_max   8
-bar_p100  8
-bar_sum   12
-bar_count 2
-
-a_on_x    1
-a_out_x   2
-b_on_y    4
-b_out_x   8
-bar_p0    2
-bar_min   2
-bar_p29   2
-bar_max   8
-bar_p100  8
-bar_sum   10
-bar_count 2
-
-a_on_x    1
-a_out_x   2
-b_in_y    4
-b_oot_x   8
-bar_p0    2
-bar_min   2
-bar_p29   2
-bar_max   4
-bar_p100  4
-bar_sum   6
-bar_count 2
-
-a_in_x    1
-a_oot_x   2
-b_on_y    4
-b_out_x   8
-bar_p0    1
-bar_min   1
-bar_p29   1
-bar_max   8
-bar_p100  8
-bar_sum   9
-bar_count 2
-
-a_in_x    1
-a_oot_x   2
-b_in_y    4
-b_oot_x   8
-bar_p0    1
-bar_min   1
-bar_p29   1
-bar_max   4
-bar_p100  4
-bar_sum   5
-bar_count 2
-
-a_in_x    1
-a_out_x   2
-b_on_y    4
-b_oot_x   8
-bar_p0    1
-bar_min   1
-bar_p29   1
-bar_max   2
-bar_p100  2
-bar_sum   3
-bar_count 2
-
-z         3
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
-
-a_in_x    1
-a_oot_x   2
-b_on_y    4
-b_oot_x   8
-bar_p0    1
-bar_min   1
-bar_p29   1
-bar_max   1
-bar_p100  1
-bar_sum   1
-bar_count 1
-
-a_on_x    1
-a_out_x   2
-b_on_y    4
-b_oot_x   8
-bar_p0    2
-bar_min   2
-bar_p29   2
-bar_max   2
-bar_p100  2
-bar_sum   2
-bar_count 1
-
-a_on_x    1
-a_oot_x   2
-b_in_y    4
-b_oot_x   8
-bar_p0    4
-bar_min   4
-bar_p29   4
-bar_max   4
-bar_p100  4
-bar_sum   4
-bar_count 1
-
-a_on_x    1
-a_oot_x   2
-b_on_y    4
-b_out_x   8
-bar_p0    8
-bar_min   8
-bar_p29   8
-bar_max   8
-bar_p100  8
-bar_sum   8
-bar_count 1
-
-z         4
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
-
-a_on_x    1
-a_oot_x   2
-b_on_y    4
-b_oot_x   8
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0011/cmd b/go/regtest/cases-pending-go-port/verb-merge-fields/0011/cmd
deleted file mode 100644
index 886d040aa..000000000
--- a/go/regtest/cases-pending-go-port/verb-merge-fields/0011/cmd
+++ /dev/null
@@ -1 +0,0 @@
-mlr --oxtab merge-fields -i -k -a p0,min,p29,max,p100,sum,count -r in_,out_       -o bar regtest/input/merge-fields-abxy.dkvp
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0011/expout b/go/regtest/cases-pending-go-port/verb-merge-fields/0011/expout
deleted file mode 100644
index 3f7a43a9a..000000000
--- a/go/regtest/cases-pending-go-port/verb-merge-fields/0011/expout
+++ /dev/null
@@ -1,227 +0,0 @@
-a_in_x    1
-a_out_x   2
-b_in_y    4
-b_out_x   8
-bar_p0    1.000000
-bar_min   1
-bar_p29   1.870000
-bar_max   8
-bar_p100  8
-bar_sum   15
-bar_count 4
-
-z         1
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
-
-a_on_x    1
-a_out_x   2
-b_in_y    4
-b_out_x   8
-bar_p0    2.000000
-bar_min   2
-bar_p29   3.160000
-bar_max   8
-bar_p100  8
-bar_sum   14
-bar_count 3
-
-a_in_x    1
-a_oot_x   2
-b_in_y    4
-b_out_x   8
-bar_p0    1.000000
-bar_min   1
-bar_p29   2.740000
-bar_max   8
-bar_p100  8
-bar_sum   13
-bar_count 3
-
-a_in_x    1
-a_out_x   2
-b_on_y    4
-b_out_x   8
-bar_p0    1.000000
-bar_min   1
-bar_p29   1.580000
-bar_max   8
-bar_p100  8
-bar_sum   11
-bar_count 3
-
-a_in_x    1
-a_out_x   2
-b_in_y    4
-b_oot_x   8
-bar_p0    1.000000
-bar_min   1
-bar_p29   1.580000
-bar_max   4
-bar_p100  4
-bar_sum   7
-bar_count 3
-
-z         2
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
-
-a_on_x    1
-a_oot_x   2
-b_in_y    4
-b_out_x   8
-bar_p0    4.000000
-bar_min   4
-bar_p29   5.160000
-bar_max   8
-bar_p100  8
-bar_sum   12
-bar_count 2
-
-a_on_x    1
-a_out_x   2
-b_on_y    4
-b_out_x   8
-bar_p0    2.000000
-bar_min   2
-bar_p29   3.740000
-bar_max   8
-bar_p100  8
-bar_sum   10
-bar_count 2
-
-a_on_x    1
-a_out_x   2
-b_in_y    4
-b_oot_x   8
-bar_p0    2.000000
-bar_min   2
-bar_p29   2.580000
-bar_max   4
-bar_p100  4
-bar_sum   6
-bar_count 2
-
-a_in_x    1
-a_oot_x   2
-b_on_y    4
-b_out_x   8
-bar_p0    1.000000
-bar_min   1
-bar_p29   3.030000
-bar_max   8
-bar_p100  8
-bar_sum   9
-bar_count 2
-
-a_in_x    1
-a_oot_x   2
-b_in_y    4
-b_oot_x   8
-bar_p0    1.000000
-bar_min   1
-bar_p29   1.870000
-bar_max   4
-bar_p100  4
-bar_sum   5
-bar_count 2
-
-a_in_x    1
-a_out_x   2
-b_on_y    4
-b_oot_x   8
-bar_p0    1.000000
-bar_min   1
-bar_p29   1.290000
-bar_max   2
-bar_p100  2
-bar_sum   3
-bar_count 2
-
-z         3
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
-
-a_in_x    1
-a_oot_x   2
-b_on_y    4
-b_oot_x   8
-bar_p0    1
-bar_min   1
-bar_p29   1
-bar_max   1
-bar_p100  1
-bar_sum   1
-bar_count 1
-
-a_on_x    1
-a_out_x   2
-b_on_y    4
-b_oot_x   8
-bar_p0    2
-bar_min   2
-bar_p29   2
-bar_max   2
-bar_p100  2
-bar_sum   2
-bar_count 1
-
-a_on_x    1
-a_oot_x   2
-b_in_y    4
-b_oot_x   8
-bar_p0    4
-bar_min   4
-bar_p29   4
-bar_max   4
-bar_p100  4
-bar_sum   4
-bar_count 1
-
-a_on_x    1
-a_oot_x   2
-b_on_y    4
-b_out_x   8
-bar_p0    8
-bar_min   8
-bar_p29   8
-bar_max   8
-bar_p100  8
-bar_sum   8
-bar_count 1
-
-z         4
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
-
-a_on_x    1
-a_oot_x   2
-b_on_y    4
-b_oot_x   8
-bar_p0    
-bar_min   
-bar_p29   
-bar_max   
-bar_p100  
-bar_sum   0
-bar_count 0
diff --git a/go/regtest/cases/cli-help/0001/expout b/go/regtest/cases/cli-help/0001/expout
index 53b5877cb..0ffd204ab 100644
--- a/go/regtest/cases/cli-help/0001/expout
+++ b/go/regtest/cases/cli-help/0001/expout
@@ -362,6 +362,50 @@ Options:
 -o {name}   Field name for output count. Default "count".
 See also "mlr most-frequent".
 ================================================================
+Usage: mlr merge-fields [options]
+Computes univariate statistics for each input record, accumulated across
+specified fields.
+Options:
+-a {sum,count,...}  Names of accumulators. One or more of:
+  count    Count instances of fields
+  mode     Find most-frequently-occurring values for fields; first-found wins tie
+  antimode Find least-frequently-occurring values for fields; first-found wins tie
+  sum      Compute sums of specified fields
+  mean     Compute averages (sample means) of specified fields
+  var      Compute sample variance of specified fields
+  stddev   Compute sample standard deviation of specified fields
+  meaneb   Estimate error bars for averages (assuming no sample autocorrelation)
+  skewness Compute sample skewness of specified fields
+  kurtosis Compute sample kurtosis of specified fields
+  min      Compute minimum values of specified fields
+  max      Compute maximum values of specified fields
+-f {a,b,c}  Value-field names on which to compute statistics. Requires -o.
+-r {a,b,c}  Regular expressions for value-field names on which to compute
+            statistics. Requires -o.
+-c {a,b,c}  Substrings for collapse mode. All fields which have the same names
+            after removing substrings will be accumulated together. Please see
+            examples below.
+-i          Use interpolated percentiles, like R's type=7; default like type=1.
+            Not sensical for string-valued fields.
+-o {name}   Output field basename for -f/-r.
+-k          Keep the input fields which contributed to the output statistics;
+            the default is to omit them.
+
+String-valued data make sense unless arithmetic on them is required,
+e.g. for sum, mean, interpolated percentiles, etc. In case of mixed data,
+numbers are less than strings.
+
+Example input data: "a_in_x=1,a_out_x=2,b_in_y=4,b_out_x=8".
+Example: mlr merge-fields -a sum,count -f a_in_x,a_out_x -o foo
+  produces "b_in_y=4,b_out_x=8,foo_sum=3,foo_count=2" since "a_in_x,a_out_x" are
+  summed over.
+Example: mlr merge-fields -a sum,count -r in_,out_ -o bar
+  produces "bar_sum=15,bar_count=4" since all four fields are summed over.
+Example: mlr merge-fields -a sum,count -c in_,out_
+  produces "a_x_sum=3,a_x_count=2,b_y_sum=4,b_y_count=1,b_x_sum=8,b_x_count=1"
+  since "a_in_x" and "a_out_x" both collapse to "a_x", "b_in_y" collapses to
+  "b_y", and "b_out_x" collapses to "b_x".
+================================================================
 Usage: mlr most-frequent [options]
 Shows the most frequently occurring distinct values for specified field names.
 The first entry is the statistical mode; the remaining are runners-up.
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0001/cmd b/go/regtest/cases/verb-merge-fields/collapse-0001/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0001/cmd
rename to go/regtest/cases/verb-merge-fields/collapse-0001/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0001/experr b/go/regtest/cases/verb-merge-fields/collapse-0001/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0001/experr
rename to go/regtest/cases/verb-merge-fields/collapse-0001/experr
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0001/expout b/go/regtest/cases/verb-merge-fields/collapse-0001/expout
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0001/expout
rename to go/regtest/cases/verb-merge-fields/collapse-0001/expout
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0002/cmd b/go/regtest/cases/verb-merge-fields/collapse-0002/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0002/cmd
rename to go/regtest/cases/verb-merge-fields/collapse-0002/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0002/experr b/go/regtest/cases/verb-merge-fields/collapse-0002/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0002/experr
rename to go/regtest/cases/verb-merge-fields/collapse-0002/experr
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0002/expout b/go/regtest/cases/verb-merge-fields/collapse-0002/expout
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0002/expout
rename to go/regtest/cases/verb-merge-fields/collapse-0002/expout
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0003/cmd b/go/regtest/cases/verb-merge-fields/collapse-0003/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0003/cmd
rename to go/regtest/cases/verb-merge-fields/collapse-0003/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0003/experr b/go/regtest/cases/verb-merge-fields/collapse-0003/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0003/experr
rename to go/regtest/cases/verb-merge-fields/collapse-0003/experr
diff --git a/go/regtest/cases/verb-merge-fields/collapse-0003/expout b/go/regtest/cases/verb-merge-fields/collapse-0003/expout
new file mode 100644
index 000000000..51ed33857
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/collapse-0003/expout
@@ -0,0 +1,4 @@
+a_p0 a_min a_p29  a_max a_p100 a_sum b_p0 b_min b_p29              b_max b_p100 b_sum
+436  436   451.66 490   490    926   195  195   267.78999999999996 446   446    641
+320  320   379.74 526   526    846   780  780   833.0699999999999  963   963    1743
+220  220   413.72 888   888    1108  705  705   741.54             831   831    1536
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0004/cmd b/go/regtest/cases/verb-merge-fields/collapse-0004/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0004/cmd
rename to go/regtest/cases/verb-merge-fields/collapse-0004/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0004/experr b/go/regtest/cases/verb-merge-fields/collapse-0004/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0004/experr
rename to go/regtest/cases/verb-merge-fields/collapse-0004/experr
diff --git a/go/regtest/cases/verb-merge-fields/collapse-0004/expout b/go/regtest/cases/verb-merge-fields/collapse-0004/expout
new file mode 100644
index 000000000..8d8d0ffd7
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/collapse-0004/expout
@@ -0,0 +1,4 @@
+a_in a_out b_in b_out a_p0 a_min a_p29  a_max a_p100 a_sum b_p0 b_min b_p29              b_max b_p100 b_sum
+436  490   446  195   436  436   451.66 490   490    926   195  195   267.78999999999996 446   446    641
+526  320   963  780   320  320   379.74 526   526    846   780  780   833.0699999999999  963   963    1743
+220  888   705  831   220  220   413.72 888   888    1108  705  705   741.54             831   831    1536
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0005/cmd b/go/regtest/cases/verb-merge-fields/collapse-0005/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0005/cmd
rename to go/regtest/cases/verb-merge-fields/collapse-0005/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0005/experr b/go/regtest/cases/verb-merge-fields/collapse-0005/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0005/experr
rename to go/regtest/cases/verb-merge-fields/collapse-0005/experr
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0005/expout b/go/regtest/cases/verb-merge-fields/collapse-0005/expout
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0005/expout
rename to go/regtest/cases/verb-merge-fields/collapse-0005/expout
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0006/cmd b/go/regtest/cases/verb-merge-fields/collapse-0006/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0006/cmd
rename to go/regtest/cases/verb-merge-fields/collapse-0006/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0006/experr b/go/regtest/cases/verb-merge-fields/collapse-0006/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0006/experr
rename to go/regtest/cases/verb-merge-fields/collapse-0006/experr
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0006/expout b/go/regtest/cases/verb-merge-fields/collapse-0006/expout
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0006/expout
rename to go/regtest/cases/verb-merge-fields/collapse-0006/expout
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0009/cmd b/go/regtest/cases/verb-merge-fields/collapse-0007/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0009/cmd
rename to go/regtest/cases/verb-merge-fields/collapse-0007/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0007/experr b/go/regtest/cases/verb-merge-fields/collapse-0007/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0007/experr
rename to go/regtest/cases/verb-merge-fields/collapse-0007/experr
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0009/expout b/go/regtest/cases/verb-merge-fields/collapse-0007/expout
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0009/expout
rename to go/regtest/cases/verb-merge-fields/collapse-0007/expout
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0012/cmd b/go/regtest/cases/verb-merge-fields/collapse-0008/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0012/cmd
rename to go/regtest/cases/verb-merge-fields/collapse-0008/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0008/experr b/go/regtest/cases/verb-merge-fields/collapse-0008/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0008/experr
rename to go/regtest/cases/verb-merge-fields/collapse-0008/experr
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0012/expout b/go/regtest/cases/verb-merge-fields/collapse-0008/expout
similarity index 95%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0012/expout
rename to go/regtest/cases/verb-merge-fields/collapse-0008/expout
index 3453b2196..48630e5b1 100644
--- a/go/regtest/cases-pending-go-port/verb-merge-fields/0012/expout
+++ b/go/regtest/cases/verb-merge-fields/collapse-0008/expout
@@ -2,9 +2,9 @@ a_in_x    1
 a_out_x   2
 b_in_y    4
 b_out_x   8
-a_x_p0    1.000000
+a_x_p0    1
 a_x_min   1
-a_x_p29   1.290000
+a_x_p29   1.29
 a_x_max   2
 a_x_p100  2
 a_x_sum   3
@@ -82,9 +82,9 @@ a_in_x    1
 a_out_x   2
 b_on_y    4
 b_out_x   8
-a_x_p0    1.000000
+a_x_p0    1
 a_x_min   1
-a_x_p29   1.290000
+a_x_p29   1.29
 a_x_max   2
 a_x_p100  2
 a_x_sum   3
@@ -101,9 +101,9 @@ a_in_x    1
 a_out_x   2
 b_in_y    4
 b_oot_x   8
-a_x_p0    1.000000
+a_x_p0    1
 a_x_min   1
-a_x_p29   1.290000
+a_x_p29   1.29
 a_x_max   2
 a_x_p100  2
 a_x_sum   3
@@ -217,9 +217,9 @@ a_in_x    1
 a_out_x   2
 b_on_y    4
 b_oot_x   8
-a_x_p0    1.000000
+a_x_p0    1
 a_x_min   1
-a_x_p29   1.290000
+a_x_p29   1.29
 a_x_max   2
 a_x_p100  2
 a_x_sum   3
diff --git a/go/regtest/cases/verb-merge-fields/name-0001/cmd b/go/regtest/cases/verb-merge-fields/name-0001/cmd
new file mode 100644
index 000000000..c17796eb9
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/name-0001/cmd
@@ -0,0 +1 @@
+mlr --oxtab head -n 2 then merge-fields -k -a p0,min,p29,max,p100,sum,count -f x,y -o xy regtest/input/abixy
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0009/experr b/go/regtest/cases/verb-merge-fields/name-0001/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0009/experr
rename to go/regtest/cases/verb-merge-fields/name-0001/experr
diff --git a/go/regtest/cases/verb-merge-fields/name-0001/expout b/go/regtest/cases/verb-merge-fields/name-0001/expout
new file mode 100644
index 000000000..85a2a7776
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/name-0001/expout
@@ -0,0 +1,25 @@
+a        pan
+b        pan
+i        1
+x        0.3467901443380824
+y        0.7268028627434533
+xy_p0    0.3467901443380824
+xy_min   0.3467901443380824
+xy_p29   0.3467901443380824
+xy_max   0.7268028627434533
+xy_p100  0.7268028627434533
+xy_sum   1.0735930070815356
+xy_count 2
+
+a        eks
+b        pan
+i        2
+x        0.7586799647899636
+y        0.5221511083334797
+xy_p0    0.5221511083334797
+xy_min   0.5221511083334797
+xy_p29   0.5221511083334797
+xy_max   0.7586799647899636
+xy_p100  0.7586799647899636
+xy_sum   1.2808310731234434
+xy_count 2
diff --git a/go/regtest/cases/verb-merge-fields/name-0002/cmd b/go/regtest/cases/verb-merge-fields/name-0002/cmd
new file mode 100644
index 000000000..6ba8a0504
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/name-0002/cmd
@@ -0,0 +1 @@
+mlr --oxtab head -n 2 then merge-fields -k -a p0,min,p29,max,p100,sum,count -f x,v -o xv regtest/input/abixy
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0010/experr b/go/regtest/cases/verb-merge-fields/name-0002/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0010/experr
rename to go/regtest/cases/verb-merge-fields/name-0002/experr
diff --git a/go/regtest/cases/verb-merge-fields/name-0002/expout b/go/regtest/cases/verb-merge-fields/name-0002/expout
new file mode 100644
index 000000000..fc0e4658b
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/name-0002/expout
@@ -0,0 +1,25 @@
+a        pan
+b        pan
+i        1
+x        0.3467901443380824
+y        0.7268028627434533
+xv_p0    0.3467901443380824
+xv_min   0.3467901443380824
+xv_p29   0.3467901443380824
+xv_max   0.3467901443380824
+xv_p100  0.3467901443380824
+xv_sum   0.3467901443380824
+xv_count 1
+
+a        eks
+b        pan
+i        2
+x        0.7586799647899636
+y        0.5221511083334797
+xv_p0    0.7586799647899636
+xv_min   0.7586799647899636
+xv_p29   0.7586799647899636
+xv_max   0.7586799647899636
+xv_p100  0.7586799647899636
+xv_sum   0.7586799647899636
+xv_count 1
diff --git a/go/regtest/cases/verb-merge-fields/name-0003/cmd b/go/regtest/cases/verb-merge-fields/name-0003/cmd
new file mode 100644
index 000000000..89d505490
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/name-0003/cmd
@@ -0,0 +1 @@
+mlr --oxtab head -n 2 then merge-fields -k -a p0,min,p29,max,p100,sum,count -f u,v -o xy regtest/input/abixy
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0011/experr b/go/regtest/cases/verb-merge-fields/name-0003/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0011/experr
rename to go/regtest/cases/verb-merge-fields/name-0003/experr
diff --git a/go/regtest/cases/verb-merge-fields/name-0003/expout b/go/regtest/cases/verb-merge-fields/name-0003/expout
new file mode 100644
index 000000000..594485cc0
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/name-0003/expout
@@ -0,0 +1,25 @@
+a        pan
+b        pan
+i        1
+x        0.3467901443380824
+y        0.7268028627434533
+xy_p0    
+xy_min   
+xy_p29   
+xy_max   
+xy_p100  
+xy_sum   0
+xy_count 0
+
+a        eks
+b        pan
+i        2
+x        0.7586799647899636
+y        0.5221511083334797
+xy_p0    
+xy_min   
+xy_p29   
+xy_max   
+xy_p100  
+xy_sum   0
+xy_count 0
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0007/cmd b/go/regtest/cases/verb-merge-fields/name-0004/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0007/cmd
rename to go/regtest/cases/verb-merge-fields/name-0004/cmd
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0012/experr b/go/regtest/cases/verb-merge-fields/name-0004/experr
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0012/experr
rename to go/regtest/cases/verb-merge-fields/name-0004/experr
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0007/expout b/go/regtest/cases/verb-merge-fields/name-0004/expout
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0007/expout
rename to go/regtest/cases/verb-merge-fields/name-0004/expout
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0010/cmd b/go/regtest/cases/verb-merge-fields/name-0005/cmd
similarity index 100%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0010/cmd
rename to go/regtest/cases/verb-merge-fields/name-0005/cmd
diff --git a/go/regtest/cases/verb-merge-fields/name-0005/experr b/go/regtest/cases/verb-merge-fields/name-0005/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/go/regtest/cases-pending-go-port/verb-merge-fields/0010/expout b/go/regtest/cases/verb-merge-fields/name-0005/expout
similarity index 93%
rename from go/regtest/cases-pending-go-port/verb-merge-fields/0010/expout
rename to go/regtest/cases/verb-merge-fields/name-0005/expout
index 11b105358..ff94a16c7 100644
--- a/go/regtest/cases-pending-go-port/verb-merge-fields/0010/expout
+++ b/go/regtest/cases/verb-merge-fields/name-0005/expout
@@ -2,9 +2,9 @@ a_in_x    1
 a_out_x   2
 b_in_y    4
 b_out_x   8
-foo_p0    1.000000
+foo_p0    1
 foo_min   1
-foo_p29   1.290000
+foo_p29   1.29
 foo_max   2
 foo_p100  2
 foo_sum   3
@@ -47,9 +47,9 @@ a_in_x    1
 a_out_x   2
 b_on_y    4
 b_out_x   8
-foo_p0    1.000000
+foo_p0    1
 foo_min   1
-foo_p29   1.290000
+foo_p29   1.29
 foo_max   2
 foo_p100  2
 foo_sum   3
@@ -59,9 +59,9 @@ a_in_x    1
 a_out_x   2
 b_in_y    4
 b_oot_x   8
-foo_p0    1.000000
+foo_p0    1
 foo_min   1
-foo_p29   1.290000
+foo_p29   1.29
 foo_max   2
 foo_p100  2
 foo_sum   3
@@ -140,9 +140,9 @@ a_in_x    1
 a_out_x   2
 b_on_y    4
 b_oot_x   8
-foo_p0    1.000000
+foo_p0    1
 foo_min   1
-foo_p29   1.290000
+foo_p29   1.29
 foo_max   2
 foo_p100  2
 foo_sum   3
diff --git a/go/regtest/cases/verb-merge-fields/regex-0001/cmd b/go/regtest/cases/verb-merge-fields/regex-0001/cmd
new file mode 100644
index 000000000..392e12410
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/regex-0001/cmd
@@ -0,0 +1 @@
+mlr merge-fields -k -a p0,min,p29,max,p100,sum,count -r in_,out_       -o bar regtest/input/merge-fields-abxy.dkvp
diff --git a/go/regtest/cases/verb-merge-fields/regex-0001/experr b/go/regtest/cases/verb-merge-fields/regex-0001/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/go/regtest/cases/verb-merge-fields/regex-0001/expout b/go/regtest/cases/verb-merge-fields/regex-0001/expout
new file mode 100644
index 000000000..dbc0bac22
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/regex-0001/expout
@@ -0,0 +1,20 @@
+a_in_x=1,a_out_x=2,b_in_y=4,b_out_x=8,bar_p0=1,bar_min=1,bar_p29=2,bar_max=8,bar_p100=8,bar_sum=15,bar_count=4
+z=1,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
+a_on_x=1,a_out_x=2,b_in_y=4,b_out_x=8,bar_p0=2,bar_min=2,bar_p29=2,bar_max=8,bar_p100=8,bar_sum=14,bar_count=3
+a_in_x=1,a_oot_x=2,b_in_y=4,b_out_x=8,bar_p0=1,bar_min=1,bar_p29=1,bar_max=8,bar_p100=8,bar_sum=13,bar_count=3
+a_in_x=1,a_out_x=2,b_on_y=4,b_out_x=8,bar_p0=1,bar_min=1,bar_p29=1,bar_max=8,bar_p100=8,bar_sum=11,bar_count=3
+a_in_x=1,a_out_x=2,b_in_y=4,b_oot_x=8,bar_p0=1,bar_min=1,bar_p29=1,bar_max=4,bar_p100=4,bar_sum=7,bar_count=3
+z=2,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
+a_on_x=1,a_oot_x=2,b_in_y=4,b_out_x=8,bar_p0=4,bar_min=4,bar_p29=4,bar_max=8,bar_p100=8,bar_sum=12,bar_count=2
+a_on_x=1,a_out_x=2,b_on_y=4,b_out_x=8,bar_p0=2,bar_min=2,bar_p29=2,bar_max=8,bar_p100=8,bar_sum=10,bar_count=2
+a_on_x=1,a_out_x=2,b_in_y=4,b_oot_x=8,bar_p0=2,bar_min=2,bar_p29=2,bar_max=4,bar_p100=4,bar_sum=6,bar_count=2
+a_in_x=1,a_oot_x=2,b_on_y=4,b_out_x=8,bar_p0=1,bar_min=1,bar_p29=1,bar_max=8,bar_p100=8,bar_sum=9,bar_count=2
+a_in_x=1,a_oot_x=2,b_in_y=4,b_oot_x=8,bar_p0=1,bar_min=1,bar_p29=1,bar_max=4,bar_p100=4,bar_sum=5,bar_count=2
+a_in_x=1,a_out_x=2,b_on_y=4,b_oot_x=8,bar_p0=1,bar_min=1,bar_p29=1,bar_max=2,bar_p100=2,bar_sum=3,bar_count=2
+z=3,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
+a_in_x=1,a_oot_x=2,b_on_y=4,b_oot_x=8,bar_p0=1,bar_min=1,bar_p29=1,bar_max=1,bar_p100=1,bar_sum=1,bar_count=1
+a_on_x=1,a_out_x=2,b_on_y=4,b_oot_x=8,bar_p0=2,bar_min=2,bar_p29=2,bar_max=2,bar_p100=2,bar_sum=2,bar_count=1
+a_on_x=1,a_oot_x=2,b_in_y=4,b_oot_x=8,bar_p0=4,bar_min=4,bar_p29=4,bar_max=4,bar_p100=4,bar_sum=4,bar_count=1
+a_on_x=1,a_oot_x=2,b_on_y=4,b_out_x=8,bar_p0=8,bar_min=8,bar_p29=8,bar_max=8,bar_p100=8,bar_sum=8,bar_count=1
+z=4,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
+a_on_x=1,a_oot_x=2,b_on_y=4,b_oot_x=8,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
diff --git a/go/regtest/cases/verb-merge-fields/regex-0002/cmd b/go/regtest/cases/verb-merge-fields/regex-0002/cmd
new file mode 100644
index 000000000..1383d7e4c
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/regex-0002/cmd
@@ -0,0 +1 @@
+mlr merge-fields -i -k -a p0,min,p29,max,p100,sum,count -r in_,out_       -o bar regtest/input/merge-fields-abxy.dkvp
diff --git a/go/regtest/cases/verb-merge-fields/regex-0002/experr b/go/regtest/cases/verb-merge-fields/regex-0002/experr
new file mode 100644
index 000000000..e69de29bb
diff --git a/go/regtest/cases/verb-merge-fields/regex-0002/expout b/go/regtest/cases/verb-merge-fields/regex-0002/expout
new file mode 100644
index 000000000..d5750408c
--- /dev/null
+++ b/go/regtest/cases/verb-merge-fields/regex-0002/expout
@@ -0,0 +1,20 @@
+a_in_x=1,a_out_x=2,b_in_y=4,b_out_x=8,bar_p0=1,bar_min=1,bar_p29=1.8699999999999999,bar_max=8,bar_p100=8,bar_sum=15,bar_count=4
+z=1,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
+a_on_x=1,a_out_x=2,b_in_y=4,b_out_x=8,bar_p0=2,bar_min=2,bar_p29=3.16,bar_max=8,bar_p100=8,bar_sum=14,bar_count=3
+a_in_x=1,a_oot_x=2,b_in_y=4,b_out_x=8,bar_p0=1,bar_min=1,bar_p29=2.7399999999999998,bar_max=8,bar_p100=8,bar_sum=13,bar_count=3
+a_in_x=1,a_out_x=2,b_on_y=4,b_out_x=8,bar_p0=1,bar_min=1,bar_p29=1.58,bar_max=8,bar_p100=8,bar_sum=11,bar_count=3
+a_in_x=1,a_out_x=2,b_in_y=4,b_oot_x=8,bar_p0=1,bar_min=1,bar_p29=1.58,bar_max=4,bar_p100=4,bar_sum=7,bar_count=3
+z=2,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
+a_on_x=1,a_oot_x=2,b_in_y=4,b_out_x=8,bar_p0=4,bar_min=4,bar_p29=5.16,bar_max=8,bar_p100=8,bar_sum=12,bar_count=2
+a_on_x=1,a_out_x=2,b_on_y=4,b_out_x=8,bar_p0=2,bar_min=2,bar_p29=3.7399999999999998,bar_max=8,bar_p100=8,bar_sum=10,bar_count=2
+a_on_x=1,a_out_x=2,b_in_y=4,b_oot_x=8,bar_p0=2,bar_min=2,bar_p29=2.58,bar_max=4,bar_p100=4,bar_sum=6,bar_count=2
+a_in_x=1,a_oot_x=2,b_on_y=4,b_out_x=8,bar_p0=1,bar_min=1,bar_p29=3.03,bar_max=8,bar_p100=8,bar_sum=9,bar_count=2
+a_in_x=1,a_oot_x=2,b_in_y=4,b_oot_x=8,bar_p0=1,bar_min=1,bar_p29=1.8699999999999999,bar_max=4,bar_p100=4,bar_sum=5,bar_count=2
+a_in_x=1,a_out_x=2,b_on_y=4,b_oot_x=8,bar_p0=1,bar_min=1,bar_p29=1.29,bar_max=2,bar_p100=2,bar_sum=3,bar_count=2
+z=3,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
+a_in_x=1,a_oot_x=2,b_on_y=4,b_oot_x=8,bar_p0=1,bar_min=1,bar_p29=1,bar_max=1,bar_p100=1,bar_sum=1,bar_count=1
+a_on_x=1,a_out_x=2,b_on_y=4,b_oot_x=8,bar_p0=2,bar_min=2,bar_p29=2,bar_max=2,bar_p100=2,bar_sum=2,bar_count=1
+a_on_x=1,a_oot_x=2,b_in_y=4,b_oot_x=8,bar_p0=4,bar_min=4,bar_p29=4,bar_max=4,bar_p100=4,bar_sum=4,bar_count=1
+a_on_x=1,a_oot_x=2,b_on_y=4,b_out_x=8,bar_p0=8,bar_min=8,bar_p29=8,bar_max=8,bar_p100=8,bar_sum=8,bar_count=1
+z=4,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
+a_on_x=1,a_oot_x=2,b_on_y=4,b_oot_x=8,bar_p0=,bar_min=,bar_p29=,bar_max=,bar_p100=,bar_sum=0,bar_count=0
diff --git a/go/src/cli/mlrcli_transformers.go b/go/src/cli/mlrcli_transformers.go
index 56bbb0b43..70f407e2e 100644
--- a/go/src/cli/mlrcli_transformers.go
+++ b/go/src/cli/mlrcli_transformers.go
@@ -39,6 +39,7 @@ var MAPPER_LOOKUP_TABLE = []transforming.TransformerSetup{
 	transformers.JoinSetup,
 	transformers.LabelSetup,
 	transformers.LeastFrequentSetup,
+	transformers.MergeFieldsSetup,
 	transformers.MostFrequentSetup,
 	transformers.NestSetup,
 	transformers.NothingSetup,
diff --git a/go/src/transformers/merge-fields.go b/go/src/transformers/merge-fields.go
new file mode 100644
index 000000000..fd305fd00
--- /dev/null
+++ b/go/src/transformers/merge-fields.go
@@ -0,0 +1,534 @@
+package transformers
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"regexp"
+	"strings"
+
+	"miller/src/cliutil"
+	"miller/src/lib"
+	"miller/src/transformers/utils"
+	"miller/src/transforming"
+	"miller/src/types"
+)
+
+// ----------------------------------------------------------------
+const verbNameMergeFields = "merge-fields"
+
+var MergeFieldsSetup = transforming.TransformerSetup{
+	Verb:         verbNameMergeFields,
+	UsageFunc:    transformerMergeFieldsUsage,
+	ParseCLIFunc: transformerMergeFieldsParseCLI,
+	IgnoresInput: false,
+}
+
+type mergeByType int
+
+const (
+	e_MERGE_BY_NAME_LIST mergeByType = iota
+	e_MERGE_BY_NAME_REGEX
+	e_MERGE_BY_COLLAPSING
+	e_MERGE_UNSPECIFIED
+)
+
+func transformerMergeFieldsUsage(
+	o *os.File,
+	doExit bool,
+	exitCode int,
+) {
+	argv0 := lib.MlrExeName()
+	verb := verbNameMergeFields
+	fmt.Fprintf(o, "Usage: %s %s [options]\n", argv0, verb)
+	fmt.Fprintf(o, "Computes univariate statistics for each input record, accumulated across\n")
+	fmt.Fprintf(o, "specified fields.\n")
+	fmt.Fprintf(o, "Options:\n")
+	fmt.Fprintf(o, "-a {sum,count,...}  Names of accumulators. One or more of:\n")
+	utils.ListStats1Accumulators(o)
+	fmt.Fprintf(o, "-f {a,b,c}  Value-field names on which to compute statistics. Requires -o.\n")
+	fmt.Fprintf(o, "-r {a,b,c}  Regular expressions for value-field names on which to compute\n")
+	fmt.Fprintf(o, "            statistics. Requires -o.\n")
+	fmt.Fprintf(o, "-c {a,b,c}  Substrings for collapse mode. All fields which have the same names\n")
+	fmt.Fprintf(o, "            after removing substrings will be accumulated together. Please see\n")
+	fmt.Fprintf(o, "            examples below.\n")
+	fmt.Fprintf(o, "-i          Use interpolated percentiles, like R's type=7; default like type=1.\n")
+	fmt.Fprintf(o, "            Not sensical for string-valued fields.\n")
+	fmt.Fprintf(o, "-o {name}   Output field basename for -f/-r.\n")
+	fmt.Fprintf(o, "-k          Keep the input fields which contributed to the output statistics;\n")
+	fmt.Fprintf(o, "            the default is to omit them.\n")
+	fmt.Fprintf(o, "\n")
+	fmt.Fprintf(o, "String-valued data make sense unless arithmetic on them is required,\n")
+	fmt.Fprintf(o, "e.g. for sum, mean, interpolated percentiles, etc. In case of mixed data,\n")
+	fmt.Fprintf(o, "numbers are less than strings.\n")
+	fmt.Fprintf(o, "\n")
+	fmt.Fprintf(o, "Example input data: \"a_in_x=1,a_out_x=2,b_in_y=4,b_out_x=8\".\n")
+	fmt.Fprintf(o, "Example: %s %s -a sum,count -f a_in_x,a_out_x -o foo\n", argv0, verb)
+	fmt.Fprintf(o, "  produces \"b_in_y=4,b_out_x=8,foo_sum=3,foo_count=2\" since \"a_in_x,a_out_x\" are\n")
+	fmt.Fprintf(o, "  summed over.\n")
+	fmt.Fprintf(o, "Example: %s %s -a sum,count -r in_,out_ -o bar\n", argv0, verb)
+	fmt.Fprintf(o, "  produces \"bar_sum=15,bar_count=4\" since all four fields are summed over.\n")
+	fmt.Fprintf(o, "Example: %s %s -a sum,count -c in_,out_\n", argv0, verb)
+	fmt.Fprintf(o, "  produces \"a_x_sum=3,a_x_count=2,b_y_sum=4,b_y_count=1,b_x_sum=8,b_x_count=1\"\n")
+	fmt.Fprintf(o, "  since \"a_in_x\" and \"a_out_x\" both collapse to \"a_x\", \"b_in_y\" collapses to\n")
+	fmt.Fprintf(o, "  \"b_y\", and \"b_out_x\" collapses to \"b_x\".\n")
+
+	if doExit {
+		os.Exit(exitCode)
+	}
+}
+
+func transformerMergeFieldsParseCLI(
+	pargi *int,
+	argc int,
+	args []string,
+	_ *cliutil.TReaderOptions,
+	__ *cliutil.TWriterOptions,
+) transforming.IRecordTransformer {
+
+	// Skip the verb name from the current spot in the mlr command line
+	argi := *pargi
+	verb := args[argi]
+	argi++
+
+	accumulatorNameList := make([]string, 0)
+	valueFieldNameList := make([]string, 0)
+	outputFieldBasename := ""
+	doWhich := e_MERGE_UNSPECIFIED
+	keepInputFields := false
+	doInterpolatedPercentiles := false
+
+	for argi < argc /* variable increment: 1 or 2 depending on flag */ {
+		opt := args[argi]
+		if !strings.HasPrefix(opt, "-") {
+			break // No more flag options to process
+		}
+		argi++
+
+		if opt == "-h" || opt == "--help" {
+			transformerMergeFieldsUsage(os.Stdout, true, 0)
+
+		} else if opt == "-a" {
+			accumulatorNameList = cliutil.VerbGetStringArrayArgOrDie(verb, opt, args, &argi, argc)
+
+		} else if opt == "-f" {
+			valueFieldNameList = cliutil.VerbGetStringArrayArgOrDie(verb, opt, args, &argi, argc)
+			doWhich = e_MERGE_BY_NAME_LIST
+
+		} else if opt == "-r" {
+			valueFieldNameList = cliutil.VerbGetStringArrayArgOrDie(verb, opt, args, &argi, argc)
+			doWhich = e_MERGE_BY_NAME_REGEX
+
+		} else if opt == "-c" {
+			valueFieldNameList = cliutil.VerbGetStringArrayArgOrDie(verb, opt, args, &argi, argc)
+			doWhich = e_MERGE_BY_COLLAPSING
+
+		} else if opt == "-o" {
+			outputFieldBasename = cliutil.VerbGetStringArgOrDie(verb, opt, args, &argi, argc)
+
+		} else if opt == "-k" {
+			keepInputFields = true
+
+		} else if opt == "-i" {
+			doInterpolatedPercentiles = true
+
+		} else if opt == "-S" {
+			// No-op pass-through for backward compatibility with Miller 5
+
+		} else if opt == "-F" {
+			// No-op pass-through for backward compatibility with Miller 5
+
+		} else {
+			transformerMergeFieldsUsage(os.Stderr, true, 1)
+		}
+	}
+
+	// TODO: libify for use across verbs.
+	if len(accumulatorNameList) == 0 {
+		fmt.Fprintf(os.Stderr, "%s %s: -a option is required.\n", lib.MlrExeName(), verbNameMergeFields)
+		fmt.Fprintf(os.Stderr, "Please see %s %s --help for more information.\n", lib.MlrExeName(), verbNameMergeFields)
+		os.Exit(1)
+	}
+	if len(valueFieldNameList) == 0 {
+		fmt.Fprintf(os.Stderr, "%s %s: -f option is required.\n", lib.MlrExeName(), verbNameMergeFields)
+		fmt.Fprintf(os.Stderr, "Please see %s %s --help for more information.\n", lib.MlrExeName(), verbNameMergeFields)
+		os.Exit(1)
+	}
+	if outputFieldBasename == "" {
+		if doWhich == e_MERGE_BY_NAME_LIST || doWhich == e_MERGE_BY_NAME_REGEX {
+			transformerMergeFieldsUsage(os.Stderr, true, 1)
+		}
+	}
+
+	transformer, _ := NewTransformerMergeFields(
+		accumulatorNameList,
+		valueFieldNameList,
+		outputFieldBasename,
+		doWhich,
+		doInterpolatedPercentiles,
+		keepInputFields,
+	)
+
+	*pargi = argi
+	return transformer
+}
+
+// ----------------------------------------------------------------
+// Given: accumulate count,sum on values x,y group by a,b.
+//
+// Example input:       Example output:
+//   a b x y            a b x_count x_sum y_count y_sum
+//   s t 1 2            s t 2       6     2       8
+//   u v 3 4            u v 1       3     1       4
+//   s t 5 6            u w 1       7     1       9
+//   u w 7 9
+//
+// Multilevel hashmap structure:
+// {
+//   "s,t" : {                <--- group-by field names
+//     "x" : {                  <--- value field name
+//       "count" : Stats1CountAccumulator object,
+//       "sum"   : Stats1SumAccumulator  object
+//     },
+//     "y" : {
+//       "count" : Stats1CountAccumulator object,
+//       "sum"   : Stats1SumAccumulator  object
+//     },
+//   },
+//   "u,v" : {
+//     "x" : {
+//       "count" : Stats1CountAccumulator object,
+//       "sum"   : Stats1SumAccumulator  object
+//     },
+//     "y" : {
+//       "count" : Stats1CountAccumulator object,
+//       "sum"   : Stats1SumAccumulator  object
+//     },
+//   },
+//   "u,w" : {
+//     "x" : {
+//       "count" : Stats1CountAccumulator object,
+//       "sum"   : Stats1SumAccumulator  object
+//     },
+//     "y" : {
+//       "count" : Stats1CountAccumulator object,
+//       "sum"   : Stats1SumAccumulator  object
+//     },
+//   },
+// }
+
+type TransformerMergeFields struct {
+	// Input:
+	accumulatorNameList       []string
+	valueFieldNameList        []string
+	outputFieldBasename       string
+	doInterpolatedPercentiles bool
+	keepInputFields           bool
+
+	// State:
+	accumulatorFactory    *utils.Stats1AccumulatorFactory
+	valueFieldNameRegexes []*regexp.Regexp
+
+	// Ordered map from accumulator name to accumulator
+	namedAccumulators *lib.OrderedMap
+
+	recordTransformerFunc transforming.RecordTransformerFunc
+}
+
+func NewTransformerMergeFields(
+	accumulatorNameList []string,
+	valueFieldNameList []string,
+	outputFieldBasename string,
+	doWhich mergeByType,
+	doInterpolatedPercentiles bool,
+	keepInputFields bool,
+) (*TransformerMergeFields, error) {
+
+	for _, accumulatorName := range accumulatorNameList {
+		if !utils.ValidateStats1AccumulatorName(accumulatorName) {
+			return nil, errors.New(
+				fmt.Sprintf(
+					"%s %s: accumulator \"%s\" not found.\n",
+					lib.MlrExeName(), verbNameMergeFields, accumulatorName,
+				),
+			)
+		}
+	}
+
+	this := &TransformerMergeFields{
+		accumulatorNameList:       accumulatorNameList,
+		valueFieldNameList:        valueFieldNameList,
+		outputFieldBasename:       outputFieldBasename,
+		doInterpolatedPercentiles: doInterpolatedPercentiles,
+		keepInputFields:           keepInputFields,
+		accumulatorFactory:        utils.NewStats1AccumulatorFactory(),
+		namedAccumulators:         lib.NewOrderedMap(),
+	}
+
+	this.valueFieldNameRegexes = make([]*regexp.Regexp, len(valueFieldNameList))
+	for i, regexString := range valueFieldNameList {
+		// Handles "a.*b"i Miller case-insensitive-regex specification
+		regex, err := lib.CompileMillerRegex(regexString)
+		if err != nil {
+			fmt.Fprintf(
+				os.Stderr,
+				"%s %s: cannot compile regex [%s]\n",
+				lib.MlrExeName(), verbNameCut, regexString,
+			)
+			os.Exit(1)
+		}
+		this.valueFieldNameRegexes[i] = regex
+	}
+
+	for _, accumulatorName := range accumulatorNameList {
+		accumulator := this.accumulatorFactory.MakeNamedAccumulator(
+			accumulatorName,
+			"", // grouping-key used for stats1, not here
+			outputFieldBasename,
+			doInterpolatedPercentiles,
+		)
+		this.namedAccumulators.Put(accumulatorName, accumulator)
+	}
+
+	if doWhich == e_MERGE_BY_NAME_LIST {
+		this.recordTransformerFunc = this.transformByNameList
+	} else if doWhich == e_MERGE_BY_NAME_REGEX {
+		this.recordTransformerFunc = this.transformByNameRegex
+	} else if doWhich == e_MERGE_BY_COLLAPSING {
+		this.recordTransformerFunc = this.transformByCollapsing
+	} else {
+		lib.InternalCodingErrorIf(true)
+	}
+
+	return this, nil
+}
+
+// ----------------------------------------------------------------
+func (this *TransformerMergeFields) Transform(
+	inrecAndContext *types.RecordAndContext,
+	outputChannel chan<- *types.RecordAndContext,
+) {
+	this.recordTransformerFunc(inrecAndContext, outputChannel)
+}
+
+// ----------------------------------------------------------------
+func (this *TransformerMergeFields) transformByNameList(
+	inrecAndContext *types.RecordAndContext,
+	outputChannel chan<- *types.RecordAndContext,
+) {
+	if inrecAndContext.EndOfStream {
+		outputChannel <- inrecAndContext // end-of-stream marker
+		return
+	}
+
+	inrec := inrecAndContext.Record
+
+	for pa := this.namedAccumulators.Head; pa != nil; pa = pa.Next {
+		accumulator := pa.Value.(*utils.Stats1NamedAccumulator)
+		accumulator.Reset() // re-use from one record to the next
+	}
+
+	for _, valueFieldName := range this.valueFieldNameList {
+		mvalue := inrec.Get(valueFieldName)
+		if mvalue == nil { // key not present
+			continue
+		}
+
+		if mvalue.IsEmpty() { // key present with empty value
+			if !this.keepInputFields {
+				inrec.Remove(valueFieldName)
+			}
+			continue
+		}
+
+		for pa := this.namedAccumulators.Head; pa != nil; pa = pa.Next {
+			accumulator := pa.Value.(*utils.Stats1NamedAccumulator)
+			accumulator.Ingest(mvalue)
+		}
+
+		if !this.keepInputFields {
+			inrec.Remove(valueFieldName)
+		}
+	}
+
+	for pa := this.namedAccumulators.Head; pa != nil; pa = pa.Next {
+		accumulator := pa.Value.(*utils.Stats1NamedAccumulator)
+		key, value := accumulator.Emit()
+		inrec.PutReference(key, value)
+	}
+
+	outputChannel <- inrecAndContext
+}
+
+// ----------------------------------------------------------------
+func (this *TransformerMergeFields) transformByNameRegex(
+	inrecAndContext *types.RecordAndContext,
+	outputChannel chan<- *types.RecordAndContext,
+) {
+	if inrecAndContext.EndOfStream {
+		outputChannel <- inrecAndContext // end-of-stream marker
+		return
+	}
+
+	inrec := inrecAndContext.Record
+
+	for pa := this.namedAccumulators.Head; pa != nil; pa = pa.Next {
+		accumulator := pa.Value.(*utils.Stats1NamedAccumulator)
+		accumulator.Reset() // re-use from one record to the next
+	}
+
+	for pe := inrec.Head; pe != nil; /* increment inside loop*/ {
+		valueFieldName := pe.Key
+
+		matched := false
+		for _, valueFieldNameRegex := range this.valueFieldNameRegexes {
+			if valueFieldNameRegex.MatchString(pe.Key) {
+				matched = true
+				break
+			}
+		}
+		if !matched {
+			pe = pe.Next
+			continue
+		}
+
+		mvalue := inrec.Get(valueFieldName)
+
+		if mvalue == nil { // Key not present
+			pe = pe.Next
+			continue
+		}
+
+		if mvalue.IsEmpty() { // Key present with empty value
+			if !this.keepInputFields { // We are modifying the record while iterating over it.
+				next := pe.Next
+				inrec.Unlink(pe)
+				pe = next
+			} else {
+				pe = pe.Next
+			}
+			continue
+		}
+
+		for pa := this.namedAccumulators.Head; pa != nil; pa = pa.Next {
+			accumulator := pa.Value.(*utils.Stats1NamedAccumulator)
+			accumulator.Ingest(mvalue)
+		}
+
+		if !this.keepInputFields { // We are modifying the record while iterating over it.
+			next := pe.Next
+			inrec.Unlink(pe)
+			pe = next
+		} else {
+			pe = pe.Next
+		}
+	}
+
+	for pa := this.namedAccumulators.Head; pa != nil; pa = pa.Next {
+		accumulator := pa.Value.(*utils.Stats1NamedAccumulator)
+		key, value := accumulator.Emit()
+		inrec.PutReference(key, value)
+	}
+
+	outputChannel <- inrecAndContext
+}
+
+// ----------------------------------------------------------------
+// mlr merge-fields -c in_,out_ -a sum
+// a_in_x  1     a_sum_x 3
+// a_out_x 2     b_sum_y 4
+// b_in_y  4     b_sum_x 8
+// b_out_x 8
+
+func (this *TransformerMergeFields) transformByCollapsing(
+	inrecAndContext *types.RecordAndContext,
+	outputChannel chan<- *types.RecordAndContext,
+) {
+	if inrecAndContext.EndOfStream {
+		outputChannel <- inrecAndContext // end-of-stream marker
+		return
+	}
+
+	inrec := inrecAndContext.Record
+	this.accumulatorFactory.Reset() // discard cached percentile-keepers
+
+	// Ordered map from short name to accumulator name to accumulator
+	collapseAccumulators := lib.NewOrderedMap()
+
+	for pe := inrec.Head; pe != nil; /* increment inside loop */ {
+		valueFieldName := pe.Key
+
+		matched := false
+		shortName := ""
+		for _, valueFieldNameRegex := range this.valueFieldNameRegexes {
+			matched = valueFieldNameRegex.MatchString(pe.Key)
+			if matched {
+				shortName = lib.RegexReplaceOnce(valueFieldNameRegex, valueFieldName, "")
+				break
+			}
+		}
+		if !matched {
+			pe = pe.Next
+			continue
+		}
+
+		mvalue := inrec.Get(valueFieldName)
+		if mvalue == nil { // Key present
+			pe = pe.Next
+			continue
+		}
+
+		if mvalue.IsEmpty() { // key present with empty value
+			if !this.keepInputFields { // We are modifying the record while iterating over it.
+				next := pe.Next
+				inrec.Unlink(pe)
+				pe = next
+			} else {
+				pe = pe.Next
+			}
+			continue
+		}
+
+		var namedAccumulators *lib.OrderedMap
+		iNamedAccumulators := collapseAccumulators.Get(shortName)
+		if iNamedAccumulators == nil {
+			namedAccumulators = lib.NewOrderedMap()
+			for _, accumulatorName := range this.accumulatorNameList {
+				accumulator := this.accumulatorFactory.MakeNamedAccumulator(
+					accumulatorName,
+					"", // grouping-key used for stats1, not here
+					shortName,
+					this.doInterpolatedPercentiles,
+				)
+				namedAccumulators.Put(accumulatorName, accumulator)
+			}
+			collapseAccumulators.Put(shortName, namedAccumulators)
+		} else {
+			namedAccumulators = iNamedAccumulators.(*lib.OrderedMap)
+		}
+
+		for pa := namedAccumulators.Head; pa != nil; pa = pa.Next {
+			accumulator := pa.Value.(*utils.Stats1NamedAccumulator)
+			accumulator.Ingest(mvalue)
+		}
+
+		if !this.keepInputFields { // We are modifying the record while iterating over it.
+			next := pe.Next
+			inrec.Unlink(pe)
+			pe = next
+		} else {
+			pe = pe.Next
+		}
+	}
+
+	for ps := collapseAccumulators.Head; ps != nil; ps = ps.Next {
+		namedAccumulators := ps.Value.(*lib.OrderedMap)
+		for pa := namedAccumulators.Head; pa != nil; pa = pa.Next {
+			accumulator := pa.Value.(*utils.Stats1NamedAccumulator)
+			key, value := accumulator.Emit()
+			inrec.PutReference(key, value)
+		}
+	}
+
+	outputChannel <- inrecAndContext
+}
diff --git a/go/src/transformers/stats1.go b/go/src/transformers/stats1.go
index 503eaa0dd..2fd196a0b 100644
--- a/go/src/transformers/stats1.go
+++ b/go/src/transformers/stats1.go
@@ -380,7 +380,7 @@ func (this *TransformerStats1) emitIntoOutputRecord(
 		for pc := level3.Head; pc != nil; pc = pc.Next {
 			namedAccumulator := pc.Value.(*utils.Stats1NamedAccumulator)
 			key, value := namedAccumulator.Emit()
-			outrec.PutCopy(key, &value)
+			outrec.PutCopy(key, value)
 		}
 	}
 }
diff --git a/go/src/transformers/utils/percentile-keeper.go b/go/src/transformers/utils/percentile-keeper.go
index 86359a276..5f8735ebe 100644
--- a/go/src/transformers/utils/percentile-keeper.go
+++ b/go/src/transformers/utils/percentile-keeper.go
@@ -29,6 +29,11 @@ func NewPercentileKeeper(
 	}
 }
 
+func (this *PercentileKeeper) Reset() {
+	this.data = make([]*types.Mlrval, 0, 1000)
+	this.sorted = false
+}
+
 // ----------------------------------------------------------------
 func (this *PercentileKeeper) Ingest(value *types.Mlrval) {
 	if len(this.data) >= cap(this.data) {
@@ -259,7 +264,7 @@ func (this *PercentileKeeper) sortIfNecessary() {
 }
 
 // ----------------------------------------------------------------
-func (this *PercentileKeeper) Emit(percentile float64) types.Mlrval {
+func (this *PercentileKeeper) Emit(percentile float64) *types.Mlrval {
 	if this.doInterpolatedPercentiles {
 		return this.EmitLinearlyInterpolated(percentile)
 	} else {
@@ -267,21 +272,21 @@ func (this *PercentileKeeper) Emit(percentile float64) types.Mlrval {
 	}
 }
 
-func (this *PercentileKeeper) EmitNonInterpolated(percentile float64) types.Mlrval {
+func (this *PercentileKeeper) EmitNonInterpolated(percentile float64) *types.Mlrval {
 	if len(this.data) == 0 {
-		return *types.MLRVAL_ABSENT
+		return types.MLRVAL_VOID
 	}
 	this.sortIfNecessary()
-	return *this.data[computeIndexNoninterpolated(int(len(this.data)), percentile)].Copy()
+	return this.data[computeIndexNoninterpolated(int(len(this.data)), percentile)].Copy()
 }
 
-func (this *PercentileKeeper) EmitLinearlyInterpolated(percentile float64) types.Mlrval {
+func (this *PercentileKeeper) EmitLinearlyInterpolated(percentile float64) *types.Mlrval {
 	if len(this.data) == 0 {
-		return *types.MLRVAL_ABSENT
+		return types.MLRVAL_VOID
 	}
 	this.sortIfNecessary()
 	output := getPercentileLinearlyInterpolated(this.data, int(len(this.data)), percentile)
-	return *output.Copy()
+	return output.Copy()
 }
 
 // ----------------------------------------------------------------
diff --git a/go/src/transformers/utils/stats1-accumulators.go b/go/src/transformers/utils/stats1-accumulators.go
index 3b0a645e9..135a2c084 100644
--- a/go/src/transformers/utils/stats1-accumulators.go
+++ b/go/src/transformers/utils/stats1-accumulators.go
@@ -16,7 +16,8 @@ import (
 // ----------------------------------------------------------------
 type IStats1Accumulator interface {
 	Ingest(value *types.Mlrval)
-	Emit() types.Mlrval
+	Emit() *types.Mlrval
+	Reset() // for merge-fields where we reset after each record instead of replace/recreate
 }
 
 // ----------------------------------------------------------------
@@ -124,10 +125,14 @@ func (this *Stats1NamedAccumulator) Ingest(value *types.Mlrval) {
 	this.accumulator.Ingest(value)
 }
 
-func (this *Stats1NamedAccumulator) Emit() (key string, value types.Mlrval) {
+func (this *Stats1NamedAccumulator) Emit() (key string, value *types.Mlrval) {
 	return this.outputFieldName, this.accumulator.Emit()
 }
 
+func (this *Stats1NamedAccumulator) Reset() {
+	this.accumulator.Reset()
+}
+
 // ----------------------------------------------------------------
 // If we are asked for p90 and p95 on the same column, we reuse the
 // percentile-keeper object to reduce runtime memory consumption.  This
@@ -195,6 +200,11 @@ func tryPercentileFromName(accumulatorName string) (float64, bool) {
 }
 
 // ----------------------------------------------------------------
+// For merge-fields wherein percentile-keepers are re-created on each record
+func (this *Stats1AccumulatorFactory) Reset() {
+	this.percentileKeepers = make(map[string]map[string]*PercentileKeeper)
+}
+
 func (this *Stats1AccumulatorFactory) MakeNamedAccumulator(
 	accumulatorName string,
 	groupingKey string,
@@ -253,7 +263,7 @@ func (this *Stats1AccumulatorFactory) MakeAccumulator(
 		// To conserve memory, percentile-keeprs on the same value-field-name
 		// (and grouping-key) are shared. For example, p25,p75 on field "x".
 		// This means though that each datapoint must be ingested only once
-		// (e.g.  by the p25 accumulator) since it shares a percentile-keepr
+		// (e.g.  by the p25 accumulator) since it shares a percentile-keeper
 		// with the p75 accumulator. We handle this by tracking the first
 		// construction.
 		return NewStats1PercentileAccumulator(percentileKeeper, percentile, isPrimary)
@@ -281,8 +291,11 @@ func NewStats1CountAccumulator() IStats1Accumulator {
 func (this *Stats1CountAccumulator) Ingest(value *types.Mlrval) {
 	this.count++
 }
-func (this *Stats1CountAccumulator) Emit() types.Mlrval {
-	return types.MlrvalFromInt(this.count)
+func (this *Stats1CountAccumulator) Emit() *types.Mlrval {
+	return types.MlrvalPointerFromInt(this.count)
+}
+func (this *Stats1CountAccumulator) Reset() {
+	this.count = 0
 }
 
 // ----------------------------------------------------------------
@@ -306,9 +319,9 @@ func (this *Stats1ModeAccumulator) Ingest(value *types.Mlrval) {
 		this.countsByValue.Put(key, iPrevious.(int)+1)
 	}
 }
-func (this *Stats1ModeAccumulator) Emit() types.Mlrval {
+func (this *Stats1ModeAccumulator) Emit() *types.Mlrval {
 	if this.countsByValue.FieldCount == 0 {
-		return *types.MLRVAL_ERROR
+		return types.MLRVAL_ERROR
 	}
 	maxValue := ""
 	var maxCount = int(0)
@@ -320,7 +333,10 @@ func (this *Stats1ModeAccumulator) Emit() types.Mlrval {
 			maxCount = count
 		}
 	}
-	return types.MlrvalFromString(maxValue)
+	return types.MlrvalPointerFromString(maxValue)
+}
+func (this *Stats1ModeAccumulator) Reset() {
+	this.countsByValue = lib.NewOrderedMap()
 }
 
 // ----------------------------------------------------------------
@@ -344,9 +360,9 @@ func (this *Stats1AntimodeAccumulator) Ingest(value *types.Mlrval) {
 		this.countsByValue.Put(key, iPrevious.(int)+1)
 	}
 }
-func (this *Stats1AntimodeAccumulator) Emit() types.Mlrval {
+func (this *Stats1AntimodeAccumulator) Emit() *types.Mlrval {
 	if this.countsByValue.FieldCount == 0 {
-		return *types.MLRVAL_ERROR
+		return types.MLRVAL_ERROR
 	}
 	minValue := ""
 	var minCount = int(0)
@@ -358,7 +374,10 @@ func (this *Stats1AntimodeAccumulator) Emit() types.Mlrval {
 			minCount = count
 		}
 	}
-	return types.MlrvalFromString(minValue)
+	return types.MlrvalPointerFromString(minValue)
+}
+func (this *Stats1AntimodeAccumulator) Reset() {
+	this.countsByValue = lib.NewOrderedMap()
 }
 
 // ----------------------------------------------------------------
@@ -374,10 +393,11 @@ func NewStats1SumAccumulator() IStats1Accumulator {
 func (this *Stats1SumAccumulator) Ingest(value *types.Mlrval) {
 	this.sum = types.MlrvalBinaryPlus(this.sum, value)
 }
-
-// xxx pending output-pointer refactor
-func (this *Stats1SumAccumulator) Emit() types.Mlrval {
-	return *this.sum.Copy()
+func (this *Stats1SumAccumulator) Emit() *types.Mlrval {
+	return this.sum.Copy()
+}
+func (this *Stats1SumAccumulator) Reset() {
+	this.sum = types.MlrvalPointerFromInt(0)
 }
 
 // ----------------------------------------------------------------
@@ -396,16 +416,17 @@ func (this *Stats1MeanAccumulator) Ingest(value *types.Mlrval) {
 	this.sum = types.MlrvalBinaryPlus(this.sum, value)
 	this.count++
 }
-
-// xxx pending output-pointer refactor
-func (this *Stats1MeanAccumulator) Emit() types.Mlrval {
+func (this *Stats1MeanAccumulator) Emit() *types.Mlrval {
 	if this.count == 0 {
-		return *types.MLRVAL_VOID
+		return types.MLRVAL_VOID
 	} else {
-		// TODO: pointer-only refactor
-		return *types.MlrvalDivide(this.sum, types.MlrvalPointerFromInt(this.count))
+		return types.MlrvalDivide(this.sum, types.MlrvalPointerFromInt(this.count))
 	}
 }
+func (this *Stats1MeanAccumulator) Reset() {
+	this.sum = types.MlrvalPointerFromInt(0)
+	this.count = 0
+}
 
 // ----------------------------------------------------------------
 type Stats1MinAccumulator struct {
@@ -420,8 +441,15 @@ func NewStats1MinAccumulator() IStats1Accumulator {
 func (this *Stats1MinAccumulator) Ingest(value *types.Mlrval) {
 	this.min = types.MlrvalBinaryMin(this.min, value)
 }
-func (this *Stats1MinAccumulator) Emit() types.Mlrval {
-	return *this.min.Copy()
+func (this *Stats1MinAccumulator) Emit() *types.Mlrval {
+	if this.min.IsAbsent() {
+		return types.MLRVAL_VOID
+	} else {
+		return this.min.Copy()
+	}
+}
+func (this *Stats1MinAccumulator) Reset() {
+	this.min = types.MLRVAL_ABSENT
 }
 
 // ----------------------------------------------------------------
@@ -437,8 +465,15 @@ func NewStats1MaxAccumulator() IStats1Accumulator {
 func (this *Stats1MaxAccumulator) Ingest(value *types.Mlrval) {
 	this.max = types.MlrvalBinaryMax(this.max, value)
 }
-func (this *Stats1MaxAccumulator) Emit() types.Mlrval {
-	return *this.max.Copy()
+func (this *Stats1MaxAccumulator) Emit() *types.Mlrval {
+	if this.max.IsAbsent() {
+		return types.MLRVAL_VOID
+	} else {
+		return this.max.Copy()
+	}
+}
+func (this *Stats1MaxAccumulator) Reset() {
+	this.max = types.MLRVAL_ABSENT
 }
 
 // ----------------------------------------------------------------
@@ -461,10 +496,14 @@ func (this *Stats1VarAccumulator) Ingest(value *types.Mlrval) {
 	this.sum = types.MlrvalBinaryPlus(this.sum, value)
 	this.sum2 = types.MlrvalBinaryPlus(this.sum2, value2)
 }
-func (this *Stats1VarAccumulator) Emit() types.Mlrval {
+func (this *Stats1VarAccumulator) Emit() *types.Mlrval {
 	mcount := types.MlrvalFromInt(this.count)
-	// TODO: pointer-only refactor
-	return *types.MlrvalGetVar(&mcount, this.sum, this.sum2)
+	return types.MlrvalGetVar(&mcount, this.sum, this.sum2)
+}
+func (this *Stats1VarAccumulator) Reset() {
+	this.count = 0
+	this.sum = types.MlrvalPointerFromInt(0)
+	this.sum2 = types.MlrvalPointerFromInt(0)
 }
 
 // ----------------------------------------------------------------
@@ -487,9 +526,14 @@ func (this *Stats1StddevAccumulator) Ingest(value *types.Mlrval) {
 	this.sum = types.MlrvalBinaryPlus(this.sum, value)
 	this.sum2 = types.MlrvalBinaryPlus(this.sum2, value2)
 }
-func (this *Stats1StddevAccumulator) Emit() types.Mlrval {
+func (this *Stats1StddevAccumulator) Emit() *types.Mlrval {
 	mcount := types.MlrvalFromInt(this.count)
-	return *types.MlrvalGetStddev(&mcount, this.sum, this.sum2)
+	return types.MlrvalGetStddev(&mcount, this.sum, this.sum2)
+}
+func (this *Stats1StddevAccumulator) Reset() {
+	this.count = 0
+	this.sum = types.MlrvalPointerFromInt(0)
+	this.sum2 = types.MlrvalPointerFromInt(0)
 }
 
 // ----------------------------------------------------------------
@@ -512,9 +556,14 @@ func (this *Stats1MeanEBAccumulator) Ingest(value *types.Mlrval) {
 	this.sum = types.MlrvalBinaryPlus(this.sum, value)
 	this.sum2 = types.MlrvalBinaryPlus(this.sum2, value2)
 }
-func (this *Stats1MeanEBAccumulator) Emit() types.Mlrval {
+func (this *Stats1MeanEBAccumulator) Emit() *types.Mlrval {
 	mcount := types.MlrvalPointerFromInt(this.count)
-	return *types.MlrvalGetMeanEB(mcount, this.sum, this.sum2)
+	return types.MlrvalGetMeanEB(mcount, this.sum, this.sum2)
+}
+func (this *Stats1MeanEBAccumulator) Reset() {
+	this.count = 0
+	this.sum = types.MlrvalPointerFromInt(0)
+	this.sum2 = types.MlrvalPointerFromInt(0)
 }
 
 // ----------------------------------------------------------------
@@ -541,10 +590,16 @@ func (this *Stats1SkewnessAccumulator) Ingest(value *types.Mlrval) {
 	this.sum2 = types.MlrvalBinaryPlus(this.sum2, value2)
 	this.sum3 = types.MlrvalBinaryPlus(this.sum3, value3)
 }
-func (this *Stats1SkewnessAccumulator) Emit() types.Mlrval {
+func (this *Stats1SkewnessAccumulator) Emit() *types.Mlrval {
 	mcount := types.MlrvalPointerFromInt(this.count)
 	return types.MlrvalGetSkewness(mcount, this.sum, this.sum2, this.sum3)
 }
+func (this *Stats1SkewnessAccumulator) Reset() {
+	this.count = 0
+	this.sum = types.MlrvalPointerFromInt(0)
+	this.sum2 = types.MlrvalPointerFromInt(0)
+	this.sum3 = types.MlrvalPointerFromInt(0)
+}
 
 // ----------------------------------------------------------------
 type Stats1KurtosisAccumulator struct {
@@ -574,10 +629,17 @@ func (this *Stats1KurtosisAccumulator) Ingest(value *types.Mlrval) {
 	this.sum3 = types.MlrvalBinaryPlus(this.sum3, value3)
 	this.sum4 = types.MlrvalBinaryPlus(this.sum4, value4)
 }
-func (this *Stats1KurtosisAccumulator) Emit() types.Mlrval {
+func (this *Stats1KurtosisAccumulator) Emit() *types.Mlrval {
 	mcount := types.MlrvalPointerFromInt(this.count)
 	return types.MlrvalGetKurtosis(mcount, this.sum, this.sum2, this.sum3, this.sum4)
 }
+func (this *Stats1KurtosisAccumulator) Reset() {
+	this.count = 0
+	this.sum = types.MlrvalPointerFromInt(0)
+	this.sum2 = types.MlrvalPointerFromInt(0)
+	this.sum3 = types.MlrvalPointerFromInt(0)
+	this.sum4 = types.MlrvalPointerFromInt(0)
+}
 
 // ----------------------------------------------------------------
 // To conserve memory, percentile-keeprs on the same value-field-name (and
@@ -609,6 +671,12 @@ func (this *Stats1PercentileAccumulator) Ingest(value *types.Mlrval) {
 	}
 }
 
-func (this *Stats1PercentileAccumulator) Emit() types.Mlrval {
+func (this *Stats1PercentileAccumulator) Emit() *types.Mlrval {
 	return this.percentileKeeper.Emit(this.percentile)
 }
+
+func (this *Stats1PercentileAccumulator) Reset() {
+	if this.isPrimary {
+		this.percentileKeeper.Reset()
+	}
+}
diff --git a/go/src/types/mlrval_functions_stats.go b/go/src/types/mlrval_functions_stats.go
index f5897d742..c6ca0f75d 100644
--- a/go/src/types/mlrval_functions_stats.go
+++ b/go/src/types/mlrval_functions_stats.go
@@ -86,11 +86,11 @@ func MlrvalGetMeanEB(mn, msum, msum2 *Mlrval) *Mlrval {
 //   = sumx2 - n mean^2
 
 // ----------------------------------------------------------------
-func MlrvalGetSkewness(mn, msum, msum2, msum3 *Mlrval) Mlrval {
+func MlrvalGetSkewness(mn, msum, msum2, msum3 *Mlrval) *Mlrval {
 	n, isInt := mn.GetIntValue()
 	lib.InternalCodingErrorIf(!isInt)
 	if n < 2 {
-		return *MLRVAL_VOID
+		return MLRVAL_VOID
 	}
 	fn := float64(n)
 	sum, isNumber := msum.GetNumericToFloatValue()
@@ -105,7 +105,7 @@ func MlrvalGetSkewness(mn, msum, msum2, msum3 *Mlrval) Mlrval {
 	numerator = numerator / fn
 	denominator := (sum2 - fn*mean*mean) / (fn - 1.0)
 	denominator = math.Pow(denominator, 1.5)
-	return MlrvalFromFloat64(numerator / denominator)
+	return MlrvalPointerFromFloat64(numerator / denominator)
 }
 
 // Unbiased:
@@ -123,11 +123,11 @@ func MlrvalGetSkewness(mn, msum, msum2, msum3 *Mlrval) Mlrval {
 //   = sumx4 - mean*(4 sumx3 - mean*(6 sumx2 - 3 n mean^2))
 
 // ----------------------------------------------------------------
-func MlrvalGetKurtosis(mn, msum, msum2, msum3, msum4 *Mlrval) Mlrval {
+func MlrvalGetKurtosis(mn, msum, msum2, msum3, msum4 *Mlrval) *Mlrval {
 	n, isInt := mn.GetIntValue()
 	lib.InternalCodingErrorIf(!isInt)
 	if n < 2 {
-		return *MLRVAL_VOID
+		return MLRVAL_VOID
 	}
 	fn := float64(n)
 	sum, isNumber := msum.GetNumericToFloatValue()
@@ -145,6 +145,6 @@ func MlrvalGetKurtosis(mn, msum, msum2, msum3, msum4 *Mlrval) Mlrval {
 	numerator = numerator / fn
 	denominator := (sum2 - fn*mean*mean) / fn
 	denominator = denominator * denominator
-	return MlrvalFromFloat64(numerator/denominator - 3.0)
+	return MlrvalPointerFromFloat64(numerator/denominator - 3.0)
 
 }
diff --git a/go/todo.txt b/go/todo.txt
index c3372d128..67a3c81c6 100644
--- a/go/todo.txt
+++ b/go/todo.txt
@@ -1,7 +1,13 @@
 ================================================================
 TOP OF LIST:
 
-! mrpl :h (and miller --help-xxx): substring ...
+!!! mlr --from s stats1 -f x,y -a corr
+  panic: runtime error: invalid memory address or nil pointer dereference
+  > needs UT
+  > needs _ -> err x all
+
+! seqgen help saying -stop not --stop
+
 ! MlrvalPointerFromString -> NewMlrvalPointerFromString et al.
 
 * blocker: regexes
diff --git a/go/u/gg b/go/u/gg
index 977137f31..1ed3c4e63 100755
--- a/go/u/gg
+++ b/go/u/gg
@@ -53,16 +53,17 @@ mention() {
 
 # ================================================================
 
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f x,y -b u
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f u,v -b x
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f z,w,s -b v
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f u,v -b nonesuch
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f u,nonesuch -b x
-
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f x,y -a u
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f u,v -a x
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f z,w,s -a v
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f u,v -a nonesuch
-run_mlr --from ../go/regtest/input/reorder.dkvp reorder -f u,nonesuch -a x
+mlr --c2p cat regtest/input/merge-fields-in-out.csv
+echo
+../c/mlr --csvlite merge-fields  -a min,max,count,sum -c _in,_out regtest/input/merge-fields-in-out.csv
+echo
+../go/mlr --csvlite merge-fields -a min,max,count,sum -c _in,_out regtest/input/merge-fields-in-out.csv
+echo
+echo
+mlr --c2p cat regtest/input/merge-fields-in-out.csv
+echo
+../c/mlr --csvlite merge-fields  -a p0,p29,p100 -c _in,_out regtest/input/merge-fields-in-out.csv
+echo
+../go/mlr --csvlite merge-fields -a p0,p29,p100 -c _in,_out regtest/input/merge-fields-in-out.csv
 
 # ================================================================