Added all doctests as unit tests.

tests/test_graph.py

 # vim: set fileencoding=utf-8 :
 import unittest
+import doctest
 
+import graph
 from node import Node, Leaf
 from graph import generate_graph
 
 
 class TestGraph(unittest.TestCase):
 
+    def test_doctest(self):
+        self.assertEqual(doctest.testmod(m=graph)[0], 0)
+
     def setUp(self):
         self.l0, self.l1, self.multi = Leaf(0), Leaf(1), Leaf('test')
 

tests/test_line.py

 import unittest
+import doctest
 
+import line
 from node import Node as N, Leaf as L
 from line import generate_line, is_id, is_int
 
 
 class TestLine(unittest.TestCase):
 
+    def test_doctest(self):
+        self.assertEqual(doctest.testmod(m=line)[0], 0)
+
     def test_empty(self):
         self.assertEquals(generate_line(None), '<empty expression>')
 

tests/test_node.py

 import unittest
+import doctest
 
+import node
 from node import Node, Leaf
 
 
 class TestNode(unittest.TestCase):
 
+    def test_doctest(self):
+        self.assertEqual(doctest.testmod(m=node)[0], 0)
+
     def setUp(self):
         self.l0 = Leaf('leaf 1')
         self.l1 = Leaf('leaf 2')