let rec write_bits b ~nbits x =
    let x = x land ((1 lsl nbits ) - 1) in (* added *)
    let n = nbits in
    if n + b.nbits >= 32 then begin
        if n > 31 then raise Bits_error;
        let n2 = 32 - b.nbits - 1 in
        let n3 = n - n2 in
        write_bits b ~nbits:n2 (x asr n3);
        write_bits b ~nbits:n3 (x land ((1 lsl n3) - 1));
    end else begin
        (* if n < 0 then raise Bits_error; *)
        (* if (x < 0 || x > (1 lsl n - 1)) && n <> 31 then raise Bits_error; *)
        b.bits <- (b.bits lsl n) lor x;
        b.nbits <- b.nbits + n;
        while b.nbits >= 8 do
            b.nbits <- b.nbits - 8;
            write_byte b.ch (b.bits asr b.nbits)
        done
    end