--- a/tests/doctest.txt	Fri Dec 14 20:47:41 2012 -0800
+++ b/tests/doctest.txt	Fri Dec 14 20:53:41 2012 -0800
@@ -48,3 +48,17 @@
     {}
     >>> sorted(graph.nodes())
     ['A', 'B', 'C']
+
+Once you set the value of a node or edge, modifying their data
+structure will not affect the graph results:
+
+    >>> values = {'a': 'b', 'c': 'easy as 1..2..3..'}
+    >>> graph['C'] = values
+    >>> graph.node('C') == values
+    True
+    >>> values['c'] = 'modifying the values'
+    >>> values['d'] = 'adding a new key'
+    >>> graph['C']['c']
+    'easy as 1..2..3..'
+    >>> 'd' in graph['C']
+    False