Added support for multi inheritance constructor calls.

node.py

@@ -3,6 +3,7 @@
 
 class Node(object):
     def __init__(self, value, *nodes):
+        super(Node, self).__init__()
         self.value, self.nodes = value, list(nodes)
 
     def __getitem__(self, n):
@@ -33,6 +34,7 @@ class Node(object):
 
 class Leaf(Node):
     def __init__(self, value):
+        super(Leaf, self).__init__(value)
         self.value = value
         self.nodes = None