human
Complete the below
def k_shell(G, k=None, core_number=None):

    ''''Return the k-shell of G.
    The k-shell is the subgraph induced by nodes with core number k.
    That is, nodes in the k-core that are not in the (k+1)-core.
    Parameters
    G : NetworkX graph
    A graph or directed graph.
    k : int, optional
    The order of the shell. If not specified return the outer shell.
    core_number : dictionary, optional
    Precomputed core numbers for the graph G.
    Returns
    G : NetworkX graph
    The k-shell subgraph
    Raises
    NetworkXError
    The k-shell is not implemented for graphs with self loops
    or parallel edges.
    Notes
    This is similar to k_corona but in that case only neighbors in the
    k-core are considered.
    Not implemented for graphs with parallel edges or self loops.
    For directed graphs the node degree is defined to be the
    in-degree + out-degree.
    Graph, node, and edge attributes are copied to the subgraph.
    See Also
    core_number
    k_corona
    References
    .. [1] A model of Internet topology using k-shell decomposition
    Shai Carmi, Shlomo Havlin, Scott Kirkpatrick, Yuval Shavitt,
    and Eran Shir, PNAS  July 3, 2007   vol. 104  no. 27  11150-11154
    http://www.pnas.org/content/104/27/11150.full'
    '''
assistant
#Here is the Python program you requested.
    def k_filter(v, k, c):
        return (c[v] == k)
    return _core_subgraph(G, k_filter, k, core_number)