Index: lib/set.rb
===================================================================
--- lib/set.rb	(revision 54238)
+++ lib/set.rb	(working copy)
@@ -696,6 +696,25 @@
   end
 end
 
+# IdentitySet implements a Set that compares members based on their identity
+# instead of their equality.
+#
+# == Example
+#
+#   require "set"
+#
+#   a_str = "a"
+#   set = IdentitySet.new([a_str, a_str, "b", "b"])
+#
+#   p set # => #<IdentitySet: {"a", "b", "b"}>
+#
+class IdentitySet < Set
+  def initialize(*args, &block)
+    @hash = Hash.new.compare_by_identity
+    super
+  end
+end
+
 module Enumerable
   # Makes a set from the enumerable object with given arguments.
   # Needs to +require "set"+ to use this method.
Index: test/test_set.rb
===================================================================
--- test/test_set.rb	(revision 54238)
+++ test/test_set.rb	(working copy)
@@ -710,6 +710,16 @@
   end
 end
 
+class TC_IdentitySet < Test::Unit::TestCase
+  def test_identityset
+    a = 'a'
+    s = IdentitySet[a, a, 'b', 'b']
+
+    assert_equal(3, s.size)
+    assert_equal([a, 'b', 'b'], s.to_a)
+  end
+end
+
 class TC_Enumerable < Test::Unit::TestCase
   def test_to_set
     ary = [2,5,4,3,2,1,3]