diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 2112fbb..d5aa79e 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -877,6 +877,7 @@
 	srand()			initialize seed used by rand()
 
 Variables:						*var-functions*
+	instanceof()		check if a variable is an instance of a given class
 	type()			type of a variable as a number
 	typename()		type of a variable as text
 	islocked()		check if a variable is locked