diff --git a/README.md b/README.md
index 036af01f3..d8ccedf2e 100644
--- a/README.md
+++ b/README.md
@@ -22,6 +22,7 @@ indices.  Examples:
 % mlr put '$attr = sub($attr, "([0-9]+)_([0-9]+)_.*", "\1:\2")' data/*.json
 % mlr stats1 -a min,mean,max,p10,p50,p90 -f flag,u,v data/*
 % mlr stats2 -a linreg-pca -f u,v -g shape data/*
+% mlr put -q '@sum[$a][$b] += $x; end {emit @sum, "a", "b"}' data/*
 ```
 
 This is something the Unix toolkit always could have done, and arguably always
diff --git a/doc/index-snippet.txt b/doc/index-snippet.txt
index 77c4aeb07..0c7a932d5 100644
--- a/doc/index-snippet.txt
+++ b/doc/index-snippet.txt
@@ -6,3 +6,4 @@
 % mlr put '$attr = sub($attr, "([0-9]+)_([0-9]+)_.*", "\1:\2")' data/*.json
 % mlr stats1 -a min,mean,max,p10,p50,p90 -f flag,u,v data/*
 % mlr stats2 -a linreg-pca -f u,v -g shape data/*
+% mlr put -q '@sum[$a][$b] += $x; end {emit @sum, "a", "b"}' data/*
diff --git a/doc/index.html b/doc/index.html
index c2e05212e..2d55b5e7e 100644
--- a/doc/index.html
+++ b/doc/index.html
@@ -158,6 +158,7 @@ positional indices. For example:
 % mlr put '$attr = sub($attr, "([0-9]+)_([0-9]+)_.*", "\1:\2")' data/*.json
 % mlr stats1 -a min,mean,max,p10,p50,p90 -f flag,u,v data/*
 % mlr stats2 -a linreg-pca -f u,v -g shape data/*
+% mlr put -q '@sum[$a][$b] += $x; end {emit @sum, "a", "b"}' data/*
 </pre>
 </div>
 <p/>