Added is_leaf attribute to Leaf and Node class.

node.py

@@ -5,6 +5,7 @@ class Node(object):
     def __init__(self, value, *nodes):
         super(Node, self).__init__()
         self.value, self.nodes = value, list(nodes)
+        self.is_leaf = False
 
     def __getitem__(self, n):
         return self.nodes[n]
@@ -37,6 +38,7 @@ class Leaf(Node):
         super(Leaf, self).__init__(value)
         self.value = value
         self.nodes = None
+        self.is_leaf = True
 
     def __len__(self):
         return len(str(self.value))