code: def _ConvertHeaderToId(header):
          
          if not (header.startswith('<') or header.endswith('>')):
              raise exceptions.BatchError(
                  'Invalid value for Content-ID: %s' % header)
          if '+' not in header:
              raise exceptions.BatchError(
                  'Invalid value for Content-ID: %s' % header)
          _, request_id = header[1:-1].rsplit('+', 1)
  
          return urllib_parse.unquote(request_id)
docs: Convert a Content-ID header value to an id.
  
          Presumes the Content-ID header conforms to the format that
          _ConvertIdToHeader() returns.
  
          Args:
            header: A string indicating the Content-ID header value.
  
          Returns:
            The extracted id value.
  
          Raises:
            BatchError if the header is not in the expected format.