Traverse through reverse relationships.

def _init_rel2dst2srcs(self, go_sources, traverse_child):
        """Traverse through reverse relationships."""
        if not traverse_child or not self.relationships:
            return {}
        rel2dst2srcs = {r:defaultdict(set) for r in self.relationships}
        goids_seen = set()
        go2obj = self.go2obj
        for goid_src in go_sources:
            goobj_src = go2obj[goid_src]
            if goid_src not in goids_seen:
                self._traverse_relationship_rev_objs(rel2dst2srcs, goobj_src, goids_seen)
        return rel2dst2srcs