def get_combo(self, symbol):
        """ get group by child symbol """
        for parent, legs in self.instrument_combos.items():
            if symbol == parent or symbol in legs.keys():
                return {
                    "parent": self.get_instrument(parent),
                    "legs": legs,
                }
        return {
            "parent": None,
            "legs": {},
        }
get group by child symbol