 + * @param {*} arg
 + */
 +util.isBoolean = function (arg) {};
++
++/**
++ * @returns {boolean}
++ * @param {*} arg
++ */
++util.isFunction = function (arg) {};
 diff --git a/vm.js b/vm.js
 index 8abe142..3e8289a 100644
 --- a/vm.js

 
 /**
diff --git a/util.js b/util.js
index ab1288b..1d72c3a 100644
--- a/util.js
+++ b/util.js
@@ -28,9 +28,6 @@

 var util = {};
 
 /**
@@ -118,3 +115,27 @@ util.pump = function(readableStream, writableStream, callback) {};
  * @param {Function} superConstructor
  */
 util.inherits = function(constructor, superConstructor) {};
