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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 _construct_post_data(self):
 """ Constructs the data structure that is required from the atlas API based on measurements, sources and times user has specified. """

        definitions = [msm.build_api_struct() for msm in self.measurements]
        probes = [source.build_api_struct() for source in self.sources]
        self.post_data = {
            "definitions": definitions,
            "probes": probes,
            "is_oneoff": self.is_oneoff
        }

        if self.is_oneoff:
            self.post_data.update({"is_oneoff": self.is_oneoff})

        if self.start_time:
            self.post_data.update(
                {"start_time": int(calendar.timegm(self.start_time.timetuple()))}
            )
        if self.stop_time:
            self.post_data.update(
                {"stop_time": int(calendar.timegm(self.stop_time.timetuple()))}
            )

        if self.bill_to:
            self.post_data.update({"bill_to": self.bill_to})