diff --git a/sh/sum b/sh/sum
index 56c1582..c8e50d0 100644
--- a/sh/sum
+++ b/sh/sum
@@ -4,6 +4,15 @@
 # YES_DASH
 
 sum() { ### @-
+    ### compute the summation of its arguments without forking processes.
+    ### this relies on the shell's built-in arithmetic operators.
+    ###
+    ### ```
+    ### $ sum 1 2 3
+    ### 6
+    ### ```
+    ###
+    ### **TODO:** consider renaming because sum(1) already exists.
     local sum=0
     for i; do
         : $((sum+=i))