--- a/python/tree.py	Tue Aug 06 18:26:42 2013 -0700
+++ b/python/tree.py	Tue Aug 06 18:30:15 2013 -0700
@@ -46,9 +46,11 @@
 
     def __init__(self, parent=None):
         self.parent = parent
+        self._children = []
 
     def children(self):
         """returns children of the tree"""
+        return self._children # base implementation
 
     def add(self, item):
         """add a child to the tree root"""