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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 ValidateChildren(self, problems):
 """Validate StopTimes and headways of this trip."""

    assert self._schedule, "Trip must be in a schedule to ValidateChildren"
    # TODO: validate distance values in stop times (if applicable)

    self.ValidateNoDuplicateStopSequences(problems)
    stoptimes = self.GetStopTimes(problems)
    stoptimes.sort(key=lambda x: x.stop_sequence)
    self.ValidateTripStartAndEndTimes(problems, stoptimes)
    self.ValidateStopTimesSequenceHasIncreasingTimeAndDistance(problems,
                                                               stoptimes)
    self.ValidateShapeDistTraveledSmallerThanMaxShapeDistance(problems,
                                                              stoptimes)
    self.ValidateDistanceFromStopToShape(problems, stoptimes)
    self.ValidateFrequencies(problems)