<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def existing_dhcp_networks(cls, conf):
 """Return a list of existing networks ids that we have configs for."""

        global _networks
        sup = super(SimpleCpnrDriver, cls)
        superkeys = sup.existing_dhcp_networks(conf)
        return set(_networks.keys()) & set(superkeys)