Return the kcrust of G.
The kcorona is the subset of vertices in the kcore which have exactly k neighbours in the kcore.
Parameters :  G : NetworkX graph
k : int
core_number : dictionary, optional


Returns :  G : NetworkX graph

Raises :  NetworkXError :

See also
Notes
Not implemented for graphs with parallel edges or self loops.
For directed graphs the node degree is defined to be the indegree + outdegree.
Graph, node, and edge attributes are copied to the subgraph.
References
[R172]  k core (bootstrap) percolation on complex networks: Critical phenomena and nonlocal effects, A. V. Goltsev, S. N. Dorogovtsev, and J. F. F. Mendes, Phys. Rev. E 73, 056101 (2006) http://link.aps.org/doi/10.1103/PhysRevE.73.056101 