diff --git a/c/containers/lhmsi.c b/c/containers/lhmsi.c
index d0075f63d..7c9d0a1ff 100644
--- a/c/containers/lhmsi.c
+++ b/c/containers/lhmsi.c
@@ -182,7 +182,21 @@ int lhmsi_get(lhmsi_t* pmap, char* key) {
 	if (pmap->states[index] == OCCUPIED)
 		return pe->value;
 	else if (pmap->states[index] == EMPTY)
-		return -999;
+		return -999; // xxx fix me ... extend the api to handle nullity.
+	else {
+		fprintf(stderr, "lhmsi_find_index_for_key did not find end of chain");
+		exit(1);
+	}
+}
+
+lhmsie_t* lhmsi_get_entry(lhmsi_t* pmap, char* key) {
+	int index = lhmsi_find_index_for_key(pmap, key);
+	lhmsie_t* pe = &pmap->entries[index];
+
+	if (pmap->states[index] == OCCUPIED)
+		return pe;
+	else if (pmap->states[index] == EMPTY)
+		return NULL;
 	else {
 		fprintf(stderr, "lhmsi_find_index_for_key did not find end of chain");
 		exit(1);
diff --git a/c/containers/lhmsi.h b/c/containers/lhmsi.h
index c5102b99d..6b39b37af 100644
--- a/c/containers/lhmsi.h
+++ b/c/containers/lhmsi.h
@@ -22,7 +22,7 @@
 typedef struct _lhmsie_t {
 	int   ideal_index;
 	char* key;
-	int   value;
+	int   value; // xxx make ull
 	struct _lhmsie_t *pprev;
 	struct _lhmsie_t *pnext;
 } lhmsie_t;
@@ -45,6 +45,7 @@ lhmsi_t* lhmsi_copy(lhmsi_t* pmap);
 void  lhmsi_free(lhmsi_t* pmap);
 void  lhmsi_put(lhmsi_t* pmap, char* key, int value);
 int   lhmsi_get(lhmsi_t* pmap, char* key);
+lhmsie_t* lhmsi_get_entry(lhmsi_t* pmap, char* key);
 int   lhmsi_has_key(lhmsi_t* pmap, char* key);
 void  lhmsi_remove(lhmsi_t* pmap, char* key);
 
diff --git a/c/containers/lhmsv.c b/c/containers/lhmsv.c
index 2195c44d8..9c31bb290 100644
--- a/c/containers/lhmsv.c
+++ b/c/containers/lhmsv.c
@@ -1,5 +1,5 @@
 // ================================================================
-// Array-only (open addressing) string-to-string linked hash map with linear
+// Array-only (open addressing) string-to-void linked hash map with linear
 // probing for collisions.
 //
 // John Kerl 2012-08-13
diff --git a/c/containers/lhmsv.h b/c/containers/lhmsv.h
index bb11fdfa9..b9650138b 100644
--- a/c/containers/lhmsv.h
+++ b/c/containers/lhmsv.h
@@ -4,7 +4,7 @@
 #include "containers/sllv.h"
 
 // ================================================================
-// Array-only (open addressing) string-to-string linked hash map with linear
+// Array-only (open addressing) string-to-void linked hash map with linear
 // probing for collisions.
 //
 // John Kerl 2012-08-13
diff --git a/c/mapping/mapper_stats1.c b/c/mapping/mapper_stats1.c
index b3739fe0b..99044e5b6 100644
--- a/c/mapping/mapper_stats1.c
+++ b/c/mapping/mapper_stats1.c
@@ -8,6 +8,7 @@
 #include "containers/slls.h"
 #include "containers/lhmslv.h"
 #include "containers/lhmsv.h"
+#include "containers/lhmsi.h"
 #include "containers/mixutil.h"
 #include "mapping/mappers.h"
 #include "cli/argparse.h"
@@ -24,6 +25,7 @@
 //   char* get();
 
 // ================================================================
+// put -> ingest/consume/something
 typedef void  acc_dput_func_t(void* pvstate, double val);
 typedef void  acc_sput_func_t(void* pvstate, char*  val);
 typedef char* acc_get_func_t (void* pvstate, char* pfree_flags);
@@ -251,6 +253,52 @@ acc_t* acc_max_alloc(static_context_t* pstatx) {
 	return pacc;
 }
 
+// ----------------------------------------------------------------
+typedef struct _acc_mode_state_t {
+	lhmsi_t* pcounts_for_value;
+	static_context_t* pstatx;
+} acc_mode_state_t;
+// mode on strings? what about "1.0" and "1" and "1.0000" ??
+void acc_mode_sput(void* pvstate, char* val) {
+	acc_mode_state_t* pstate = pvstate;
+	lhmsie_t* pe = lhmsi_get_entry(pstate->pcounts_for_value, val);
+	if (pe == NULL) {
+		// xxx at the moment, lhmsi does a strdup so we needn't.
+		lhmsi_put(pstate->pcounts_for_value, val, 1);
+	} else {
+		pe->value++;
+	}
+}
+char* acc_mode_get(void* pvstate, char* pfree_flags) {
+	acc_mode_state_t* pstate = pvstate;
+	int max_count = 0;
+	char* max_key = NULL;
+	for (lhmsie_t* pe = pstate->pcounts_for_value->phead; pe != NULL; pe = pe->pnext) {
+		int count = pe->value;
+		if (count > max_count) {
+			max_key = pe->key;
+			max_count = count;
+		}
+	}
+	return max_key;
+}
+// xxx somewhere note a crucial assumption: while pctx is passed in
+// to the mapper on a per-row basis, we stash it here on first use and
+// use it on subsequent rows. assumptions:
+// * the address doesn't change
+// * the content we use (namely, ofmt) isn't row-dependent
+acc_t* acc_mode_alloc(static_context_t* pstatx) {
+	acc_t* pacc = mlr_malloc_or_die(sizeof(acc_t));
+	acc_mode_state_t* pstate = mlr_malloc_or_die(sizeof(acc_mode_state_t));
+	pstate->pcounts_for_value = lhmsi_alloc();
+	pstate->pstatx   = pstatx;
+	pacc->pvstate    = (void*)pstate;
+	pacc->psput_func = &acc_mode_sput;
+	pacc->pdput_func = NULL;
+	pacc->pget_func  = &acc_mode_get;
+	return pacc;
+}
+
 // ----------------------------------------------------------------
 typedef struct _acc_lookup_t {
 	char* name;
@@ -264,6 +312,7 @@ static acc_lookup_t acc_lookup_table[] = {
 	{"avgeb",  acc_avgeb_alloc},
 	{"min",    acc_min_alloc},
 	{"max",    acc_max_alloc},
+	{"mode",   acc_mode_alloc},
 };
 static int acc_lookup_table_length = sizeof(acc_lookup_table) / sizeof(acc_lookup_table[0]);
 
@@ -281,7 +330,7 @@ typedef struct _mapper_stats1_state_t {
 	slls_t* pvalue_field_names;
 	slls_t* pgroup_by_field_names;
 
-	lhmslv_t* pmaps_level_1;
+	lhmslv_t* groups;
 
 } mapper_stats1_state_t;
 
@@ -293,10 +342,10 @@ typedef struct _mapper_stats1_state_t {
 //   s t 5 6            u w 1       7     1       9
 //   u w 7 9
 
-// ["s","t"] |--> "x" |--> "sum" |--> acc_sum_t* (as void*)
+// ["s","t"] |--> "x" |--> "sum" |--> acc_t* (as void*)
 // level_1      level_2   level_3
 // lhmslv_t     lhmsv_t   lhmsv_t
-// acc_sum_t implements interface:
+// acc_t implements interface:
 //   void  init();
 //   void  dacc(double dval);
 //   void  sacc(char*  sval);
@@ -323,10 +372,10 @@ sllv_t* mapper_stats1_func(lrec_t* pinrec, context_t* pctx, void* pvstate) {
 			return NULL;
 		}
 
-		lhmsv_t* pmaps_level_2 = lhmslv_get(pstate->pmaps_level_1, pgroup_by_field_values);
-		if (pmaps_level_2 == NULL) {
-			pmaps_level_2 = lhmsv_alloc();
-			lhmslv_put(pstate->pmaps_level_1, slls_copy(pgroup_by_field_values), pmaps_level_2);
+		lhmsv_t* group_to_acc_field = lhmslv_get(pstate->groups, pgroup_by_field_values);
+		if (group_to_acc_field == NULL) {
+			group_to_acc_field = lhmsv_alloc();
+			lhmslv_put(pstate->groups, slls_copy(pgroup_by_field_values), group_to_acc_field);
 		}
 
 		sllse_t* pa = pstate->pvalue_field_names->phead;
@@ -338,17 +387,17 @@ sllv_t* mapper_stats1_func(lrec_t* pinrec, context_t* pctx, void* pvstate) {
 			int   have_dval = FALSE;
 			double value_field_dval = -999.0;
 
-			lhmsv_t* pmaps_level_3 = lhmsv_get(pmaps_level_2, value_field_name);
-			if (pmaps_level_3 == NULL) {
-				pmaps_level_3 = lhmsv_alloc();
-				lhmsv_put(pmaps_level_2, value_field_name, pmaps_level_3);
+			lhmsv_t* acc_field_to_acc_state = lhmsv_get(group_to_acc_field, value_field_name);
+			if (acc_field_to_acc_state == NULL) {
+				acc_field_to_acc_state = lhmsv_alloc();
+				lhmsv_put(group_to_acc_field, value_field_name, acc_field_to_acc_state);
 			}
 
 			// for "sum", "count"
 			sllse_t* pc = pstate->paccumulator_names->phead;
 			for ( ; pc != NULL; pc = pc->pnext) {
 				char* acc_name = pc->value;
-				acc_t* pacc = lhmsv_get(pmaps_level_3, acc_name);
+				acc_t* pacc = lhmsv_get(acc_field_to_acc_state, acc_name);
 				if (pacc == NULL) {
 					pacc = make_acc(acc_name, &pctx->statx);
 					if (pacc == NULL) {
@@ -356,7 +405,7 @@ sllv_t* mapper_stats1_func(lrec_t* pinrec, context_t* pctx, void* pvstate) {
 							acc_name);
 						exit(1);
 					}
-					lhmsv_put(pmaps_level_3, acc_name, pacc);
+					lhmsv_put(acc_field_to_acc_state, acc_name, pacc);
 				}
 				if (pacc == NULL) {
 					// xxx needs argv[0] somewhere ...
@@ -383,7 +432,7 @@ sllv_t* mapper_stats1_func(lrec_t* pinrec, context_t* pctx, void* pvstate) {
 	else {
 		sllv_t* poutrecs = sllv_alloc();
 
-		for (lhmslve_t* pa = pstate->pmaps_level_1->phead; pa != NULL; pa = pa->pnext) {
+		for (lhmslve_t* pa = pstate->groups->phead; pa != NULL; pa = pa->pnext) {
 			lrec_t* poutrec = lrec_unbacked_alloc();
 
 			slls_t* pgroup_by_field_values = pa->key;
@@ -396,13 +445,13 @@ sllv_t* mapper_stats1_func(lrec_t* pinrec, context_t* pctx, void* pvstate) {
 			}
 
 			// Add in fields such as x_sum=#, y_count=#, etc.:
-			lhmsv_t* pmaps_level_2 = pa->value;
+			lhmsv_t* group_to_acc_field = pa->value;
 			// for "x", "y"
-			for (lhmsve_t* pd = pmaps_level_2->phead; pd != NULL; pd = pd->pnext) {
+			for (lhmsve_t* pd = group_to_acc_field->phead; pd != NULL; pd = pd->pnext) {
 				char* value_field_name = pd->key;
-				lhmsv_t* pmaps_level_3 = pd->value;
+				lhmsv_t* acc_field_to_acc_state = pd->value;
 				// for "count", "sum"
-				for (lhmsve_t* pe = pmaps_level_3->phead; pe != NULL; pe = pe->pnext) {
+				for (lhmsve_t* pe = acc_field_to_acc_state->phead; pe != NULL; pe = pe->pnext) {
 					char* acc_name = pe->key;
 					acc_t* pacc = pe->value;
 
@@ -427,7 +476,7 @@ static void mapper_stats1_free(void* pvstate) {
 	slls_free(pstate->pvalue_field_names);
 	slls_free(pstate->pgroup_by_field_names);
 	// xxx free the level-2's 1st
-	lhmslv_free(pstate->pmaps_level_1);
+	lhmslv_free(pstate->groups);
 }
 
 mapper_t* mapper_stats1_alloc(slls_t* paccumulator_names, slls_t* pvalue_field_names, slls_t* pgroup_by_field_names) {
@@ -438,7 +487,7 @@ mapper_t* mapper_stats1_alloc(slls_t* paccumulator_names, slls_t* pvalue_field_n
 	pstate->paccumulator_names    = paccumulator_names;
 	pstate->pvalue_field_names    = pvalue_field_names;
 	pstate->pgroup_by_field_names = pgroup_by_field_names;
-	pstate->pmaps_level_1         = lhmslv_alloc();
+	pstate->groups                = lhmslv_alloc();
 
 	pmapper->pvstate              = pstate;
 	pmapper->pmapper_process_func = mapper_stats1_func;
diff --git a/c/t1.rb b/c/t1.rb
index 9ead58959..43eb3e957 100755
--- a/c/t1.rb
+++ b/c/t1.rb
@@ -77,7 +77,7 @@ run('CORRMLRV',  "mlr      --csv  stats2 -a corr -f x,y -g a,b < data/big.csv >
 puts
 puts
 
-run("sortsort1", "sort -t, -k 1,2      < data/big > /dev/null")
+run("sortsort1", "sort -t= -k 1,2      < data/big > /dev/null")
 run("SORTMLR1",  "mlr sort -f a,b      < data/big > /dev/null")
 puts
 
@@ -90,5 +90,5 @@ run("sortsort1v",  "sort -t, -k 1,4            < data/big.csv > /dev/null")
 run("SORTMLR1V",   "mlr      --csv sort -f a,x < data/big.csv > /dev/null")
 puts
 
-run("sortsort2v",  "sort -t, -k 4,5            < data/big.csv > /dev/null")
+run("sortsort2v",  "sort -t, -n -k 4,5         < data/big.csv > /dev/null")
 run("SORTMLR2V",   "mlr      --csv sort -n x,y < data/big.csv > /dev/null")
diff --git a/c/test/expected/out b/c/test/expected/out
index 4d61a3de5..90161d541 100644
--- a/c/test/expected/out
+++ b/c/test/expected/out
@@ -402,18 +402,18 @@ a=zee,b=wye,count=2
 a=hat,b=wye,count=2
 a=pan,b=wye,count=2
 
-./test/../mlr --opprint stats1 -a avg,sum,count,min,max -f i,x,y -g a,b ./test/input/abixy
-a   b   i_avg     i_sum     i_count i_min     i_max     x_avg    x_sum    x_count x_min    x_max    y_avg    y_sum    y_count y_min    y_max
-pan pan 1.000000  1.000000  1       1.000000  1.000000  0.346790 0.346790 1       0.346790 0.346790 0.726803 0.726803 1       0.726803 0.726803
-eks pan 2.000000  2.000000  1       2.000000  2.000000  0.758680 0.758680 1       0.758680 0.758680 0.522151 0.522151 1       0.522151 0.522151
-wye wye 3.000000  3.000000  1       3.000000  3.000000  0.204603 0.204603 1       0.204603 0.204603 0.338319 0.338319 1       0.338319 0.338319
-eks wye 4.000000  4.000000  1       4.000000  4.000000  0.381399 0.381399 1       0.381399 0.381399 0.134189 0.134189 1       0.134189 0.134189
-wye pan 5.000000  5.000000  1       5.000000  5.000000  0.573289 0.573289 1       0.573289 0.573289 0.863624 0.863624 1       0.863624 0.863624
-zee pan 6.000000  6.000000  1       6.000000  6.000000  0.527126 0.527126 1       0.527126 0.527126 0.493221 0.493221 1       0.493221 0.493221
-eks zee 7.000000  7.000000  1       7.000000  7.000000  0.611784 0.611784 1       0.611784 0.611784 0.187885 0.187885 1       0.187885 0.187885
-zee wye 8.000000  8.000000  1       8.000000  8.000000  0.598554 0.598554 1       0.598554 0.598554 0.976181 0.976181 1       0.976181 0.976181
-hat wye 9.000000  9.000000  1       9.000000  9.000000  0.031442 0.031442 1       0.031442 0.031442 0.749551 0.749551 1       0.749551 0.749551
-pan wye 10.000000 10.000000 1       10.000000 10.000000 0.502626 0.502626 1       0.502626 0.502626 0.952618 0.952618 1       0.952618 0.952618
+./test/../mlr --opprint stats1 -a avg,sum,count,min,max,mode -f i,x,y -g a,b ./test/input/abixy
+a   b   i_avg     i_sum     i_count i_min     i_max     i_mode x_avg    x_sum    x_count x_min    x_max    x_mode              y_avg    y_sum    y_count y_min    y_max    y_mode
+pan pan 1.000000  1.000000  1       1.000000  1.000000  1      0.346790 0.346790 1       0.346790 0.346790 0.3467901443380824  0.726803 0.726803 1       0.726803 0.726803 0.7268028627434533
+eks pan 2.000000  2.000000  1       2.000000  2.000000  2      0.758680 0.758680 1       0.758680 0.758680 0.7586799647899636  0.522151 0.522151 1       0.522151 0.522151 0.5221511083334797
+wye wye 3.000000  3.000000  1       3.000000  3.000000  3      0.204603 0.204603 1       0.204603 0.204603 0.20460330576630303 0.338319 0.338319 1       0.338319 0.338319 0.33831852551664776
+eks wye 4.000000  4.000000  1       4.000000  4.000000  4      0.381399 0.381399 1       0.381399 0.381399 0.38139939387114097 0.134189 0.134189 1       0.134189 0.134189 0.13418874328430463
+wye pan 5.000000  5.000000  1       5.000000  5.000000  5      0.573289 0.573289 1       0.573289 0.573289 0.5732889198020006  0.863624 0.863624 1       0.863624 0.863624 0.8636244699032729
+zee pan 6.000000  6.000000  1       6.000000  6.000000  6      0.527126 0.527126 1       0.527126 0.527126 0.5271261600918548  0.493221 0.493221 1       0.493221 0.493221 0.49322128674835697
+eks zee 7.000000  7.000000  1       7.000000  7.000000  7      0.611784 0.611784 1       0.611784 0.611784 0.6117840605678454  0.187885 0.187885 1       0.187885 0.187885 0.1878849191181694
+zee wye 8.000000  8.000000  1       8.000000  8.000000  8      0.598554 0.598554 1       0.598554 0.598554 0.5985540091064224  0.976181 0.976181 1       0.976181 0.976181 0.976181385699006
+hat wye 9.000000  9.000000  1       9.000000  9.000000  9      0.031442 0.031442 1       0.031442 0.031442 0.03144187646093577 0.749551 0.749551 1       0.749551 0.749551 0.7495507603507059
+pan wye 10.000000 10.000000 1       10.000000 10.000000 10     0.502626 0.502626 1       0.502626 0.502626 0.5026260055412137  0.952618 0.952618 1       0.952618 0.952618 0.9526183602969864
 
 ./test/../mlr --opprint stats2 -a linreg-ols,linreg-pca,r2,corr,cov -f x,y,xy,y2,x2,x2 -g a,b ./test/input/abixy-wide
 a   b   x_y_ols_m x_y_ols_b x_y_pca_m x_y_pca_b x_y_pca_quality x_y_r2   x_y_corr  x_y_cov   xy_y2_ols_m xy_y2_ols_b xy_y2_pca_m xy_y2_pca_b xy_y2_pca_quality xy_y2_r2 xy_y2_corr xy_y2_cov x2_x2_ols_m x2_x2_ols_b x2_x2_pca_m x2_x2_pca_b x2_x2_pca_quality x2_x2_r2 x2_x2_corr x2_x2_cov
diff --git a/c/test/output/out b/c/test/output/out
index 4d61a3de5..90161d541 100644
--- a/c/test/output/out
+++ b/c/test/output/out
@@ -402,18 +402,18 @@ a=zee,b=wye,count=2
 a=hat,b=wye,count=2
 a=pan,b=wye,count=2
 
-./test/../mlr --opprint stats1 -a avg,sum,count,min,max -f i,x,y -g a,b ./test/input/abixy
-a   b   i_avg     i_sum     i_count i_min     i_max     x_avg    x_sum    x_count x_min    x_max    y_avg    y_sum    y_count y_min    y_max
-pan pan 1.000000  1.000000  1       1.000000  1.000000  0.346790 0.346790 1       0.346790 0.346790 0.726803 0.726803 1       0.726803 0.726803
-eks pan 2.000000  2.000000  1       2.000000  2.000000  0.758680 0.758680 1       0.758680 0.758680 0.522151 0.522151 1       0.522151 0.522151
-wye wye 3.000000  3.000000  1       3.000000  3.000000  0.204603 0.204603 1       0.204603 0.204603 0.338319 0.338319 1       0.338319 0.338319
-eks wye 4.000000  4.000000  1       4.000000  4.000000  0.381399 0.381399 1       0.381399 0.381399 0.134189 0.134189 1       0.134189 0.134189
-wye pan 5.000000  5.000000  1       5.000000  5.000000  0.573289 0.573289 1       0.573289 0.573289 0.863624 0.863624 1       0.863624 0.863624
-zee pan 6.000000  6.000000  1       6.000000  6.000000  0.527126 0.527126 1       0.527126 0.527126 0.493221 0.493221 1       0.493221 0.493221
-eks zee 7.000000  7.000000  1       7.000000  7.000000  0.611784 0.611784 1       0.611784 0.611784 0.187885 0.187885 1       0.187885 0.187885
-zee wye 8.000000  8.000000  1       8.000000  8.000000  0.598554 0.598554 1       0.598554 0.598554 0.976181 0.976181 1       0.976181 0.976181
-hat wye 9.000000  9.000000  1       9.000000  9.000000  0.031442 0.031442 1       0.031442 0.031442 0.749551 0.749551 1       0.749551 0.749551
-pan wye 10.000000 10.000000 1       10.000000 10.000000 0.502626 0.502626 1       0.502626 0.502626 0.952618 0.952618 1       0.952618 0.952618
+./test/../mlr --opprint stats1 -a avg,sum,count,min,max,mode -f i,x,y -g a,b ./test/input/abixy
+a   b   i_avg     i_sum     i_count i_min     i_max     i_mode x_avg    x_sum    x_count x_min    x_max    x_mode              y_avg    y_sum    y_count y_min    y_max    y_mode
+pan pan 1.000000  1.000000  1       1.000000  1.000000  1      0.346790 0.346790 1       0.346790 0.346790 0.3467901443380824  0.726803 0.726803 1       0.726803 0.726803 0.7268028627434533
+eks pan 2.000000  2.000000  1       2.000000  2.000000  2      0.758680 0.758680 1       0.758680 0.758680 0.7586799647899636  0.522151 0.522151 1       0.522151 0.522151 0.5221511083334797
+wye wye 3.000000  3.000000  1       3.000000  3.000000  3      0.204603 0.204603 1       0.204603 0.204603 0.20460330576630303 0.338319 0.338319 1       0.338319 0.338319 0.33831852551664776
+eks wye 4.000000  4.000000  1       4.000000  4.000000  4      0.381399 0.381399 1       0.381399 0.381399 0.38139939387114097 0.134189 0.134189 1       0.134189 0.134189 0.13418874328430463
+wye pan 5.000000  5.000000  1       5.000000  5.000000  5      0.573289 0.573289 1       0.573289 0.573289 0.5732889198020006  0.863624 0.863624 1       0.863624 0.863624 0.8636244699032729
+zee pan 6.000000  6.000000  1       6.000000  6.000000  6      0.527126 0.527126 1       0.527126 0.527126 0.5271261600918548  0.493221 0.493221 1       0.493221 0.493221 0.49322128674835697
+eks zee 7.000000  7.000000  1       7.000000  7.000000  7      0.611784 0.611784 1       0.611784 0.611784 0.6117840605678454  0.187885 0.187885 1       0.187885 0.187885 0.1878849191181694
+zee wye 8.000000  8.000000  1       8.000000  8.000000  8      0.598554 0.598554 1       0.598554 0.598554 0.5985540091064224  0.976181 0.976181 1       0.976181 0.976181 0.976181385699006
+hat wye 9.000000  9.000000  1       9.000000  9.000000  9      0.031442 0.031442 1       0.031442 0.031442 0.03144187646093577 0.749551 0.749551 1       0.749551 0.749551 0.7495507603507059
+pan wye 10.000000 10.000000 1       10.000000 10.000000 10     0.502626 0.502626 1       0.502626 0.502626 0.5026260055412137  0.952618 0.952618 1       0.952618 0.952618 0.9526183602969864
 
 ./test/../mlr --opprint stats2 -a linreg-ols,linreg-pca,r2,corr,cov -f x,y,xy,y2,x2,x2 -g a,b ./test/input/abixy-wide
 a   b   x_y_ols_m x_y_ols_b x_y_pca_m x_y_pca_b x_y_pca_quality x_y_r2   x_y_corr  x_y_cov   xy_y2_ols_m xy_y2_ols_b xy_y2_pca_m xy_y2_pca_b xy_y2_pca_quality xy_y2_r2 xy_y2_corr xy_y2_cov x2_x2_ols_m x2_x2_ols_b x2_x2_pca_m x2_x2_pca_b x2_x2_pca_quality x2_x2_r2 x2_x2_corr x2_x2_cov
diff --git a/c/test/run b/c/test/run
index c9e1db8d2..edc8bc54c 100755
--- a/c/test/run
+++ b/c/test/run
@@ -102,7 +102,7 @@ run_command $mlr sort -f a -r b -nf x -nr y $indir/abixy
 announce STATS
 run_command $mlr count-distinct -f a,b $indir/small $indir/abixy
 
-run_command $mlr --opprint stats1 -a avg,sum,count,min,max -f i,x,y -g a,b $indir/abixy
+run_command $mlr --opprint stats1 -a avg,sum,count,min,max,mode -f i,x,y -g a,b $indir/abixy
 run_command $mlr --opprint stats2 -a linreg-ols,linreg-pca,r2,corr,cov -f x,y,xy,y2,x2,x2 -g a,b $indir/abixy-wide
 run_command $mlr --opprint step -a rsum,delta,counter -f x,y -g a $indir/abixy
 run_command $mlr --opprint histogram -f x,y --lo 0 --hi 1 --nbins 20 $indir/small
diff --git a/c/todo.txt b/c/todo.txt
index d5ce14894..e725116dc 100644
--- a/c/todo.txt
+++ b/c/todo.txt
@@ -6,6 +6,9 @@
 ================================================================
 FEATURES
 !! quantiles !!
+   -> be sure to include p99/p50 example (with then-chaining) in mlrwik
+!! mode
+
 !! reminder pgr legend is broken !!
   http://en.wikipedia.org/wiki/Order_statistic_tree
 !! dkvp as generalization of nidx. restructure mlrwik to emphasize this.
@@ -18,7 +21,6 @@ FEATURES
 ! ordered cut (a la reorder). either a new command (yeck) or cut option (e.g. cut -o)
 * stats1 mode: lhmsi & then sort. what about "1"=="1.0"?
 * mod op (either c-like, or sane) and put into wikidoc if so.
-* impl/doc mlr check: no output
 * linreg-quality 2nd pass -- code it up in stats2 w/ -m {m} -b {b} -- ?
 
 ================================================================
@@ -66,6 +68,7 @@ TESTING
 * reorder reg-test cases
 * ut cases: multifile & multimapper often. e.g. wrapping everything in cat then X then cat ...
 * ut cases: make should-fail machinery & use it for null-key dkvp cases.
+* ut case for stats1 mode
 
 ================================================================
 DOC
diff --git a/doc/content-for-reference.html b/doc/content-for-reference.html
index 45dc9585a..e226fa9a4 100644
--- a/doc/content-for-reference.html
+++ b/doc/content-for-reference.html
@@ -89,6 +89,8 @@ Miller. For example:
 
 POKI_CARDIFY(mlr cut --complement -f os_version *.dat | mlr sort -f hostname,uptime)HERE
 
+POKI_INCLUDE_ESCAPED(data/then-chaining-performance.txt)HERE
+
 <p/>
 For better performance (avoiding redundant string-parsing and string-formatting
 when you pipe Miller commands together) you can, if you like, instead simply
@@ -449,6 +451,49 @@ POKI_RUN_COMMAND{{mlr --oxtab stats1 -a count,sum,avg -f x,y data/medium}}HERE
 POKI_RUN_COMMAND{{mlr --opprint stats1 -a avg -f x,y -g b then sort -f b data/medium}}HERE
 </td></tr></table>
 
+<!--
+kerl@kerl-mbp[s1j0d1][c]$ mlr --opprint count-distinct -f b then sort -nr count data/small
+b   count
+wye 5
+pan 4
+zee 1
+kerl@kerl-mbp[s0j0d1][c]$ mlr --opprint count-distinct -f a then sort -nr count data/small
+a   count
+eks 3
+pan 2
+wye 2
+zee 2
+hat 1
+kerl@kerl-mbp[s0j0d1][c]$ mlr --omlr --opprint stats1 -a mode -f a,b data/small
+a_mode b_mode
+eks    wye
+kerl@kerl-mbp[s0j0d1][c]$ mlr --opprint sort -f a data/small
+a   b   i  x                   y
+eks pan 2  0.7586799647899636  0.5221511083334797
+eks wye 4  0.38139939387114097 0.13418874328430463
+eks zee 7  0.6117840605678454  0.1878849191181694
+hat wye 9  0.03144187646093577 0.7495507603507059
+pan pan 1  0.3467901443380824  0.7268028627434533
+pan wye 10 0.5026260055412137  0.9526183602969864
+wye wye 3  0.20460330576630303 0.33831852551664776
+wye pan 5  0.5732889198020006  0.8636244699032729
+zee pan 6  0.5271261600918548  0.49322128674835697
+zee wye 8  0.5985540091064224  0.976181385699006
+kerl@kerl-mbp[s0j0d1][c]$ mlr --opprint sort -f b data/small
+a   b   i  x                   y
+pan pan 1  0.3467901443380824  0.7268028627434533
+eks pan 2  0.7586799647899636  0.5221511083334797
+wye pan 5  0.5732889198020006  0.8636244699032729
+zee pan 6  0.5271261600918548  0.49322128674835697
+wye wye 3  0.20460330576630303 0.33831852551664776
+eks wye 4  0.38139939387114097 0.13418874328430463
+zee wye 8  0.5985540091064224  0.976181385699006
+hat wye 9  0.03144187646093577 0.7495507603507059
+pan wye 10 0.5026260055412137  0.9526183602969864
+eks zee 7  0.6117840605678454  0.1878849191181694
+
+-->
+
 <!-- ================================================================ -->
 <h2>stats2</h2>
 
diff --git a/doc/data/then-chaining-performance.txt b/doc/data/then-chaining-performance.txt
new file mode 100644
index 000000000..4dd2ea5b2
--- /dev/null
+++ b/doc/data/then-chaining-performance.txt
@@ -0,0 +1,16 @@
+% cat piped.sh
+mlr cut -x -f i,y data/big | mlr sort -n y > /dev/null
+
+% time sh piped.sh
+real 0m2.828s
+user 0m3.183s
+sys  0m0.137s
+
+
+% cat chained.sh
+mlr cut -x -f i,y then sort -n y data/big > /dev/null
+
+% time sh chained.sh
+real 0m2.082s
+user 0m1.933s
+sys  0m0.137s
diff --git a/doc/reference.html b/doc/reference.html
index 6b212ccc1..53681f86e 100644
--- a/doc/reference.html
+++ b/doc/reference.html
@@ -264,6 +264,29 @@ mlr cut --complement -f os_version *.dat | mlr sort -f hostname,uptime
 </div>
 <p/>
 
+<p/>
+<div class="pokipanel">
+<pre>
+% cat piped.sh
+mlr cut -x -f i,y data/big | mlr sort -n y &gt; /dev/null
+
+% time sh piped.sh
+real 0m2.828s
+user 0m3.183s
+sys  0m0.137s
+
+
+% cat chained.sh
+mlr cut -x -f i,y then sort -n y data/big &gt; /dev/null
+
+% time sh chained.sh
+real 0m2.082s
+user 0m1.933s
+sys  0m0.137s
+</pre>
+</div>
+<p/>
+
 <p/>
 For better performance (avoiding redundant string-parsing and string-formatting
 when you pipe Miller commands together) you can, if you like, instead simply
@@ -1240,6 +1263,49 @@ zee 0.504242 0.502997
 <p/>
 </td></tr></table>
 
+<!--
+kerl@kerl-mbp[s1j0d1][c]$ mlr --opprint count-distinct -f b then sort -nr count data/small
+b   count
+wye 5
+pan 4
+zee 1
+kerl@kerl-mbp[s0j0d1][c]$ mlr --opprint count-distinct -f a then sort -nr count data/small
+a   count
+eks 3
+pan 2
+wye 2
+zee 2
+hat 1
+kerl@kerl-mbp[s0j0d1][c]$ mlr --omlr --opprint stats1 -a mode -f a,b data/small
+a_mode b_mode
+eks    wye
+kerl@kerl-mbp[s0j0d1][c]$ mlr --opprint sort -f a data/small
+a   b   i  x                   y
+eks pan 2  0.7586799647899636  0.5221511083334797
+eks wye 4  0.38139939387114097 0.13418874328430463
+eks zee 7  0.6117840605678454  0.1878849191181694
+hat wye 9  0.03144187646093577 0.7495507603507059
+pan pan 1  0.3467901443380824  0.7268028627434533
+pan wye 10 0.5026260055412137  0.9526183602969864
+wye wye 3  0.20460330576630303 0.33831852551664776
+wye pan 5  0.5732889198020006  0.8636244699032729
+zee pan 6  0.5271261600918548  0.49322128674835697
+zee wye 8  0.5985540091064224  0.976181385699006
+kerl@kerl-mbp[s0j0d1][c]$ mlr --opprint sort -f b data/small
+a   b   i  x                   y
+pan pan 1  0.3467901443380824  0.7268028627434533
+eks pan 2  0.7586799647899636  0.5221511083334797
+wye pan 5  0.5732889198020006  0.8636244699032729
+zee pan 6  0.5271261600918548  0.49322128674835697
+wye wye 3  0.20460330576630303 0.33831852551664776
+eks wye 4  0.38139939387114097 0.13418874328430463
+zee wye 8  0.5985540091064224  0.976181385699006
+hat wye 9  0.03144187646093577 0.7495507603507059
+pan wye 10 0.5026260055412137  0.9526183602969864
+eks zee 7  0.6117840605678454  0.1878849191181694
+
+-->
+
 <!-- ================================================================ -->
 <a id="stats2"/><h2>stats2</h2>