Removed parent attribute to avoid cyclic references between nodes.

node.py

@@ -4,10 +4,6 @@
 class Node(object):
     def __init__(self, value, *nodes):
         self.value, self.nodes = value, list(nodes)
-        self.parent = None
-
-        for a in self.nodes:
-            a.parent = self
 
     def __getitem__(self, n):
         return self.nodes[n]
@@ -38,7 +34,6 @@ class Node(object):
 class Leaf(Node):
     def __init__(self, value):
         self.value = value
-        self.parent = None
         self.nodes = None
 
     def __len__(self):