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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _next_timestamp(self, now, last):
 """ Returns the timestamp that should be used if ``now`` is the current time and ``last`` is the last timestamp returned by this object. Intended for internal and testing use only; to generate timestamps, call an instantiated ``MonotonicTimestampGenerator`` object. :param int now: an integer to be used as the current time, typically representing the current time in microseconds since the UNIX epoch :param int last: an integer representing the last timestamp returned by this object """

        if now > last:
            self.last = now
            return now
        else:
            self._maybe_warn(now=now)
            self.last = last + 1
            return self.last