Deletes to_par_edge and nd_id. To be used when nd_id is an out-degree= 1 node

def suppress_deg_one_node(self, to_par_edge, nd_id, to_child_edge):
        """Deletes to_par_edge and nd_id. To be used when nd_id is an out-degree= 1 node"""
        # circumvent the node with nd_id
        to_child_edge_id = to_child_edge['@id']
        par = to_par_edge['@source']
        self._edge_by_source[par][to_child_edge_id] = to_child_edge
        to_child_edge['@source'] = par
        # make it a tip...
        del self._edge_by_source[nd_id]
        # delete it
        self._del_tip(nd_id)