diff --git a/doc/concept.sh b/doc/concept.sh
index b5d3e88..6e48cea 100644
--- a/doc/concept.sh
+++ b/doc/concept.sh
@@ -167,8 +167,8 @@ pxl|pixel|canv|canvas draw
 # - args with &-prefix modify corresponding var in global scope
 
 # the function scope "myfunction1" starts here
-# argument $1 is readonly; $2 is read/write
-def myfunction1 $1 &$2
+# argument $1 is readonly; $2 is read/write; $3 and $4 are optional and will default to `null`
+def myfunction1 $1 &$2 ($3 $4)
 
 # current function scope ends here
 end