diff --git a/lib/set.js b/lib/set.js
new file mode 100644
index 0000000..4a9dabf
--- /dev/null
+++ b/lib/set.js
@@ -0,0 +1,66 @@
+// batteries
+// Copyright 2010 - 2011 Sami Samhuri <sami@samhuri.net>
+
+exports.Set = Set;
+
+exports.extendNative = function() {
+  global.Set = Set;
+};
+
+var ownProps = Object.getOwnPropertyNames;
+
+function Set(items) {
+  if (!Array.isArray(items)) items = [].slice.call(arguments);
+  var n = 0;
+  this.members = items.reduce(function(set, x) {
+    if (!(x in set)) {
+      n += 1;
+      set[x] = x;
+    }
+    return set;
+  }, Object.create(null));
+  this.size = n;
+}
+
+Set.prototype.add = function(item) {
+  if (!(item in this.members)) {
+    this.members[item] = item;
+    this.size += 1;
+  }
+};
+
+Set.prototype.clear = function() {
+  this.members = Object.create(null);
+  this.size = 0;
+};
+
+Set.prototype.contains = function(item) {
+  return item in this.members;
+}
+
+Set.prototype.diff = function(other) {
+  var d = []
+    , x
+    ;
+  for (x in this.members) if (!(x in other.members)) d.push(x);
+  return new Set(d);
+};
+
+Set.prototype.isEmpty = function() {
+  return this.size === 0;
+};
+
+Set.prototype.remove = function(item) {
+  if (item in this.members) {
+    delete this.members[item];
+    this.size -= 1;
+  }
+};
+
+Set.prototype.toArray = function() {
+  return Object.keys(this.members);
+};
+
+Set.prototype.union = function(other) {
+  return new Set(ownProps(this.members).concat(ownProps(other.members)));
+};