<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def mock(self, slot, rpc_id, value):
 """Store a mock return value for an RPC Args: slot (SlotIdentifier):
 The slot we are mocking rpc_id (int):
 The rpc we are mocking value (int):
 The value that should be returned when the RPC is called. """


        address = slot.address

        if address not in self.mock_rpcs:
            self.mock_rpcs[address] = {}

        self.mock_rpcs[address][rpc_id] = value