<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_predecessors(self, head):
 """Given a head set of nodes, get a list of edges of which the node set is the head of each edge. :param head: set of nodes that correspond to the heads of some (possibly empty) set of edges. :returns: set -- hyperedge_ids of the hyperedges that have head in the head. """

        frozen_head = frozenset(head)
        # If this node set isn't any head in the hypergraph, then it has
        # no predecessors; thus, return an empty list
        if frozen_head not in self._predecessors:
            return set()

        return set(self._predecessors[frozen_head].values())