Traverse from source GO up parents.

def _traverse_parent_objs(self, goobj_child):
        """Traverse from source GO up parents."""
        child_id = goobj_child.id
        # mark child as seen
        self.seen_cids.add(child_id)
        self.godag.go2obj[child_id] = goobj_child
        # Loop through parents of child object
        for parent_obj in goobj_child.parents:
            parent_id = parent_obj.id
            self.godag.p_from_cs[parent_id].add(child_id)
            # If parent has not been seen, traverse
            if parent_id not in self.seen_cids:
                self._traverse_parent_objs(parent_obj)