docs: def _offset_of_next_ff_byte(self, start):
          """
          Return the offset of the next '\xFF' byte in *stream* starting with
          the byte at offset *start*. Returns *start* if the byte at that
          offset is a hex 255; it does not necessarily advance in the stream.
          """
          self._stream.seek(start)
          byte_ = self._read_byte()
          while byte_ != b'\xFF':
              byte_ = self._read_byte()
          offset_of_ff_byte = self._stream.tell() - 1
          return offset_of_ff_byte