transitive closure graph has 0 edge
>>> g = Graph()
>>> n1 = g.add_vertex()
>>> n2 = g.add_vertex()
>>> e = g.add_edge(n1,n2)
>>> g
<Graph object, directed, with 2 vertices and 1 edge at 0x36920d0>
>>> tc = transitive_closure(g)
<Graph object, directed, with 2 vertices and 0 edges at 0x38599d0>
>>> tc.reindex_edges()
>>> tc
<Graph object, directed, with 2 vertices and 1 edge at 0x38599d0>