correct implementation of difference

set.js

@@ -101,11 +101,22 @@ Set.prototype.intersect = function(iset){
   return oset
 }
 
+Set.prototype.symmetricDifference = function(iset){
+  // (A \ B) ∪ (B \ A)
+  return this.difference(iset).union(iset.difference(this));
+}
+
 Set.prototype.difference = function(iset){
+  /* From http://en.wikipedia.org/wiki/Set_theory
+   *   Set difference of U and A, denoted U \ A, is the set of all 
+   *   members of U that are not members of A. The set difference 
+   *   {1,2,3} \ {2,3,4} is {1} , while, conversely, the set difference
+   *   {2,3,4} \ {1,2,3} is {4}
+   */
   var items = iset.get()
     , i = 0
     , l = items.length
-    , oset = this.union(iset)
+    , oset = new Set(this.get())
     , prop
 
   for(; i < l; i++){

test/set-test.js

@@ -190,6 +190,19 @@ vows.describe('Set').addBatch({
                 assert.instanceOf(topic, Set)
             }
 
+            , "will get an array of [0,1]": function(topic){
+                assert.deepEqual(topic.get(), [0,1])
+            }
+        }
+        , "when taking the symmetric difference with a Set of [2,3,4,5,6,7]": {
+              topic: function(topic){
+                return topic.symmetricDifference(new Set([2,3,4,5,6,7]))
+              }
+
+            , "will give me a set": function(topic){
+                assert.instanceOf(topic, Set)
+            }
+
             , "will get an array of [0,1,5,6,7]": function(topic){
                 assert.deepEqual(topic.get(), [0,1,5,6,7])
             }