diff --git a/src/UnitJS/Fun.js b/src/UnitJS/Fun.js
index a0868ae..de78c8e 100644
--- a/src/UnitJS/Fun.js
+++ b/src/UnitJS/Fun.js
@@ -3,6 +3,7 @@ return {
 	compose: compose,
 	defined: defined,
 	id: id,
+	not: not,
 	insert: insert,
 	map: map,
 	mapFilter: mapFilter,
@@ -32,6 +33,10 @@ function id(x) {
 	return x;
 }
 
+function not(x) {
+	return !x;
+}
+
 function insert(obj, t, comparer, min, max) {
 	min = defined(min) ? min : 0;
 	max = defined(max) ? max : t.length;