code: public boolean contains(T transaction, I item) {
  		int t = transactionToIndex(transaction);
  		if (t < 0)
  			return false;
  		int i = itemToIndex(item);
  		if (i < 0)
  			return false;
  		return matrix.contains(t, i);
  	}
docs: Checks if the given transaction-item pair is contained within the set
  
  @param transaction
  the transaction of the pair
  @param item
  the item of the pair
  @return <code>true</code> if the given transaction-item pair is contained
  within the set