code: private void expandTo(int wordIndex) {
  		int wordsRequired = wordIndex+1;
  		if (wordsInUse < wordsRequired) {
  			ensureCapacity(wordsRequired);
  			wordsInUse = wordsRequired;
  		}
  	}
docs: Ensures that the BitSet can accommodate a given wordIndex,
  temporarily violating the invariants.  The caller must
  restore the invariants before returning to the user,
  possibly using recalculateWordsInUse().
  @param	wordIndex the index to be accommodated.