def _retrieve_start_nodes(net, mc):
    """
    retrieve nodes with in_degree == 0 as starting points for main path analysis
    args:
        net: a CitationNetwork object
        mc: minimum citations
    returns:
        list of start nodes in net
    """
    return [node for node in net.nodes if (net.in_degree(node) == 0 and net.out_degree(node) > mc)]