code: public LR0ItemSet calc(LR0ItemSet itemSet, Construction construction) {
  	Map<Construction, LR0ItemSet> map = gotos.get(itemSet);
  	if (map == null) {
  	    return calculate(itemSet, construction);
  	}
  	LR0ItemSet result = map.get(construction);
  	if (result == null) {
  	    return calculate(itemSet, construction);
  	}
  	return result;
      }
docs: This is the closure method for a set of items. This method is descibed in
  the Dragon Book 4.6.2.
  
  @param itemSet
  is an {@link LR0ItemSet} to be used for calculation.
  @param construction
  is the construction to be used.
  @return A complete set of items is returned containing the parameter
  items and all calculated extensions.