code: private boolean verifyGhostBitsClear() {
  		for (int i = numWords; i < bits.length; i++) {
  			if (bits[i] != 0)
  				return false;
  		}
  
  		if ((numBits & 0x3f) == 0)
  			return true;
  
  		long mask = -1L << numBits;
  
  		return (bits[numWords - 1] & mask) == 0;
  	}
docs: Checks if the bits past numBits are clear. Some methods rely on this
  implicit assumption: search for "Depends on the ghost bits being clear!"
  
  @return true if the bits past numBits are clear.