Return a list of nodes in topological sort order.
A topological sort is a nonunique permutation of the nodes such that an edge from u to v implies that u appears before v in the topological sort order.
Parameters :  G : NetworkX digraph
nbunch : container of nodes (optional)


Raises :  NetworkXError :
NetworkXUnfeasible :

See also
Notes
This algorithm is based on a description and proof in The Algorithm Design Manual [R177] .
References
[R177]  (1, 2) Skiena, S. S. The Algorithm Design Manual (SpringerVerlag, 1998). http://www.amazon.com/exec/obidos/ASIN/0387948600/ref=ase_thealgorithmrepo/ 