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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 AddFile(self, filepath):
 """Adds a file path as a source. Args: filepath: a string representing a path to the file. Returns: True if the file is not an already existing source. """

    if filepath not in self._files:
      self._files.add(filepath)
      return True
    return False