Fix warning for recursive assignments.

diff --git a/data2.py b/data2.py
index b0f5654..f4231c2 100644
--- a/data2.py
+++ b/data2.py
@@ -27,7 +27,7 @@ class Set(object):
   def __init__(self,*elems):
     self._list = list(elems)

-  def __in__(self,obj):
+  def __contains__(self,obj):
     return str(obj) in map(str,self._list)

   def add(self,obj):