Ooooo mighty graph algorithm class, pleeeaase let me fix

a tiny bug that has no comparision to the greaaatness of 
thyself.
I humbly bow on thy greatness, aaaaalll mighty one and 
the only one.

pisi/graph.py

@@ -46,7 +46,7 @@ class Digraph(object):
         "return a list of edge descriptors"
         list = []
         for u in self.__v:
-            for v in self.__u:
+            for v in self.__adj[u]:
                 list.append( (u,v) )
         return list