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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 _SetCompleted(self):
 """Atomically marks the breakpoint as completed. Returns: True if the breakpoint wasn't marked already completed or False if the breakpoint was already completed. """

    with self._lock:
      if self._completed:
        return False
      self._completed = True
      return True