<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def register_combo(self, parent, legs):
 """ add contracts to groups """

        parent = self.ibConn.contractString(parent)
        legs_dict = {}
        for leg in legs:
            leg = self.ibConn.contractString(leg)
            legs_dict[leg] = self.get_instrument(leg)
        self.instrument_combos[parent] = legs_dict