code: def _init_rel2dst2srcs(self, go_sources, traverse_child):
          
          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
docs: Traverse through reverse relationships.