Set self._sink_scc_labels, which is a list containing the labels of
        the strongly connected components.

def _find_sink_scc(self):
        """
        Set self._sink_scc_labels, which is a list containing the labels of
        the strongly connected components.

        """
        condensation_lil = self._condensation_lil()

        # A sink SCC is a SCC such that none of its members is strongly
        # connected to nodes in other SCCs
        # Those k's such that graph_condensed_lil.rows[k] == []
        self._sink_scc_labels = \
            np.where(np.logical_not(condensation_lil.rows))[0]