code: public boolean retainAll(Collection<T> trans, I item) {
  		if (isEmpty())
  			return false;
  		if (trans == null || trans.isEmpty() || item == null) {
  			clear();
  			return true;
  		}
  		return matrix.retainAll(allTransactions.convert(trans).indices(), itemToIndex(item));
  	}
docs: Retains the pairs obtained from the Cartesian product of transactions and
  items
  
  @param trans
  collection of transactions
  @param item
  the item
  @return <code>true</code> if the set set has been changed