Traverse up parents.

def _init_p2cs(self, go_sources, traverse_parent):
        """Traverse up parents."""
        if not traverse_parent:
            return {}
        p2cs = defaultdict(set)
        goids_seen = set()
        go2obj = self.go2obj
        for goid_src in go_sources:
            goobj_src = go2obj[goid_src]
            if goid_src not in goids_seen:
                ##F self._traverse_parent_objs(p2cs, goobj_src, go2obj, goids_seen)
                self._traverse_parent_objs(p2cs, goobj_src, goids_seen)
        return p2cs