Issue #225, adding _.union, _.difference, to complement _.without and _.intersection.

underscore.js

@@ -327,8 +327,7 @@
 
   // Return a version of the array that does not contain the specified value(s).
   _.without = function(array) {
-    var values = slice.call(arguments, 1);
-    return _.filter(array, function(value){ return !_.include(values, value); });
+    return _.difference(array, slice.call(arguments, 1));
   };
 
   // Produce a duplicate-free version of the array. If the array has already
@@ -341,9 +340,15 @@
     }, []);
   };
 
+  // Produce an array that contains the union: each distinct element from all of
+  // the passed-in arrays.
+  _.union = function() {
+    return _.uniq(_.flatten(arguments));
+  };
+
   // Produce an array that contains every item shared between all the
-  // passed-in arrays.
-  _.intersect = function(array) {
+  // passed-in arrays. (Aliased as "intersect" for back-compat.)
+  _.intersection = _.intersect = function(array) {
     var rest = slice.call(arguments, 1);
     return _.filter(_.uniq(array), function(item) {
       return _.every(rest, function(other) {
@@ -352,6 +357,12 @@
     });
   };
 
+  // Take the difference between one array and another.
+  // Only the elements present in just the first array will remain.
+  _.difference = function(array, other) {
+    return _.filter(array, function(value){ return !_.include(other, value); });
+  };
+
   // Zip together multiple lists into a single array -- elements that share
   // an index go together.
   _.zip = function() {