Return all parent GO IDs through both reverse 'is_a' and all relationships.

def get_all_lower(self):
        """Return all parent GO IDs through both reverse 'is_a' and all relationships."""
        all_lower = set()
        for lower in self.get_goterms_lower():
            all_lower.add(lower.item_id)
            all_lower |= lower.get_all_lower()
        return all_lower