class BinData::IO::Write

def writebits(val, nbits, endian)

the bits are to be stored in +:big+ or +:little+ endian format.
Writes +nbits+ bits from +val+ to the stream. +endian+ specifies whether 
def writebits(val, nbits, endian)
  if @wendian != endian
    # don't mix bits of differing endian
    flushbits
    @wendian = endian
  end
  clamped_val = val & mask(nbits)
  if endian == :big
    write_big_endian_bits(clamped_val, nbits)
  else
    write_little_endian_bits(clamped_val, nbits)
  end
end