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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 _next_file_gen(self, root):
 """Generator for next file element in the document. Args: root: root element of the XML tree. Yields: GCSFileStat for the next file. """

    for e in root.getiterator(common._T_CONTENTS):
      st_ctime, size, etag, key = None, None, None, None
      for child in e.getiterator('*'):
        if child.tag == common._T_LAST_MODIFIED:
          st_ctime = common.dt_str_to_posix(child.text)
        elif child.tag == common._T_ETAG:
          etag = child.text
        elif child.tag == common._T_SIZE:
          size = child.text
        elif child.tag == common._T_KEY:
          key = child.text
      yield common.GCSFileStat(self._path + '/' + key,
                               size, etag, st_ctime)
      e.clear()
    yield None