sclass BitHead > ByteHead {
  gettable int align;
  gettable int currentByte;
  
  replae SomethingIO with BitIO.
  
  *() {}
  *(InputStream inputStream) { super(inputStream); }
  *(OutputStream outputStream) { super(outputStream); }

  // slow version of write byte[] looping manually
  void write(byte[] data) {
    for (b : data) write(b);
  }
  
  void writeByte(int i) {
    if (align == 0)
      super.writeByte(i);
    else {
      currentByte |= i << align;
      super.writeByte(currentByte);
      currentByte = i >> (8-align);
    }
  }
  
  void writeBit(int i) {
    writeBit((i & 1) != 0);
  }
  
  void writeBit(bool b) {
    if (b) currentByte |= 1 << align;
    if (align == 7) {
      super.writeByte(currentByte);
      currentByte = align = 0;
    } else
      ++align;
  }
  
  bool byteAligned() {
    ret align == 0;
  }
  
  void completeByte aka flushBits aka finish(bool padWithOnes default false) {
    if (byteAligned()) ret;
    if (padWithOnes) currentByte |= 0xFF << align;
    super.writeByte(currentByte);
    currentByte = align = 0;
  }
  
  void writeTrailingBitCount aka trailingBitCount(bool padWithOnes default false) {
    int bitCount = modRange_incl(align(), 1, 8);
    completeByte(padWithOnes);
    writeByte(bitCount);
  }
  
  void exchange(SomethingIO writable) {
    if (writable != null) writable.readWrite(this);
  }
}