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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 start(self):
 """Starts the external measurement program."""

        assert not self.is_running(), 'Attempted to start an energy measurement while one was already running.'

        self._measurement_process = subprocess.Popen(
            [self._executable, '-r'],
            stdout=subprocess.PIPE,
            stderr=subprocess.PIPE,
            bufsize=10000,
            preexec_fn=os.setpgrp, # Prevent delivery of Ctrl+C to subprocess
            )