diff --git a/src/_ps b/src/_ps
index a7e6be6..dd8ba5c 100644
--- a/src/_ps
+++ b/src/_ps
@@ -1,6 +1,18 @@
 #compdef ps
 
-# This works with procps version 3.2.8 
+# This works with procps version 3.2.8
+
+# This depends on the _comma_separated function, which should be available from the same
+# place you got this file. If it isn't then you can uncomment the following lines to define
+# the function:
+#
+# _comma_separated()
+# {
+#     compset -P '*,'
+#     local -a used vals
+#     used=(${(s:,:)IPREFIX})
+#     _wanted tty expl $1 compadd -S ',' -q -F used "$@[2,-1]"
+# }
 
 local context state state_descr line
 typeset -A opt_args