Added negation to node stringification.

node.py

@@ -32,7 +32,8 @@ class Node(object):
         return copy
 
     def __str__(self):
-        return '<Node value=%s nodes=%s>' % (str(self.value), str(self.nodes))
+        return '<Node value=%s nodes=%s negated=%d>' \
+               % (str(self.value), str(self.nodes), self.negated)
 
     def title(self):
         return str(self.value)
@@ -52,4 +53,4 @@ class Leaf(Node):
         return repr(self.value)
 
     def __str__(self):
-        return str(self.value)
+        return '-' * self.negated + str(self.value)

tests/test_node.py

@@ -34,3 +34,11 @@ class TestNode(unittest.TestCase):
     def test_title(self):
         assert self.node.title() == 'node'
         assert self.l0.title() == 'leaf 1'
+
+    def test____str__(self):
+        l1 = Leaf(1)
+        self.assertEqual(str(l1), '1')
+        l1 = -l1
+        self.assertEqual(str(l1), '-1')
+        l1 = -l1
+        self.assertEqual(str(l1), '--1')