docs: def _next_non_ff_byte(self, start):
          """
          Return an offset, byte 2-tuple for the next byte in *stream* that is
          not '\xFF', starting with the byte at offset *start*. If the byte at
          offset *start* is not '\xFF', *start* and the returned *offset* will
          be the same.
          """
          self._stream.seek(start)
          byte_ = self._read_byte()
          while byte_ == b'\xFF':
              byte_ = self._read_byte()
          offset_of_non_ff_byte = self._stream.tell() - 1
          return offset_of_non_ff_byte, byte_