<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def is_extent_valid(self, start_time, duration, flag=None):
 """Check if the duration contains any non-valid frames Parameters start_time: int Beginning of the duration to check in gps seconds duration: int Number of seconds after the start_time to check flag: str, optional Override the default valid mask with a user defined mask. Returns ------- status: boolean Returns True if all of the status information if valid, False if any is not. """

        sr = self.raw_buffer.sample_rate
        s = int((start_time - self.raw_buffer.start_time) * sr)
        e = s + int(duration * sr) + 1
        data = self.raw_buffer[s:e]
        return self.check_valid(data, flag=flag)