<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _request_check(self, address):
 """Wake one monitor. Hold the lock when calling this."""

        server = self._servers.get(address)

        # "server" is None if another thread removed it from the topology.
        if server:
            server.request_check()