class BinData::IO

def read_big_endian_bits(nbits)
  while @rnbits < nbits
    accumulate_big_endian_bits
  end
  val     = (@rval >> (@rnbits - nbits)) & mask(nbits)
  @rnbits -= nbits
  @rval   &= mask(@rnbits)
  val
end