code: public LR0ItemSet calc(LR0ItemSet initialItemSet) {
  	LR0ItemSet result = closures.get(initialItemSet);
  	if (result == null) {
  	    return calculate(initialItemSet);
  	}
  	return result;
      }
docs: This is the closure method for a set of items. This method is described
  in the Dragon Book 4.6.2.
  
  @param initialItemSet
  is the {@link LR0ItemSet} to be used.
  @return A complete set of items is returned containing the parameter
  items and all calculated extensions.