code: private long diode(long offset, long length) {
  		int lb = (int) (64 - offset);
  		int rb = (int) (64 - (offset + length));
  		return (-1L << lb) ^ (-1L << rb);
  	}
docs: a diode is a long value whose left and right margin are ZERO, while middle
  bits are ONE in binary string layout. it looks like a diode in shape.
  
  @param offset
  left margin position
  @param length
  offset+length is right margin position
  @return a long value