Moved magic methods from ExpressionNode to Node.

node.py

@@ -2,8 +2,12 @@
 
 
 class Node(object):
-    def __init__(self, label, *nodes):
-        self.label, self.nodes = label, list(nodes)
+    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]
@@ -18,12 +22,19 @@ class Node(object):
         return len(self.nodes)
 
     def title(self):
-        return str(self.label)
+        return str(self.value)
 
 
 class Leaf(object):
-    def __init__(self, label):
-        self.label = label
+    def __init__(self, value):
+        self.value = value
+        self.parent = None
+
+    def __repr__(self):
+        return repr(self.value)
+
+    def __str__(self):
+        return str(self.value)
 
     def title(self):
-        return str(self.label)
+        return str(self.value)