Sometimes, a value of some attribute depends on the realtion between two attributes. In case of cashpoint system such situation can also be identified:
The system requires a comparison beetween the PIN number that has been entered by the user and the PIN number that is stored on the card in order to check if the user has entered a correct PIN. Having three attributes: enteredPIN, correctPIN and authorized we can simply compare the first two of them and depending on a result we can set a value of the third one:

• if enteredPIN = correctPIN then authorized := true;
• if enteredPIN ≠ correctPIN then authorized := false;

The XTT method does not allow to direct comparison of two attributes within conditional part of a rule. So, you cannot create the rules presented above. If you want to comapre the values of two attributes you have to introduce a third, supplementary attribute that will hold the value, which corresponds to the result of comparison. Then, depending on the value of this (supplementary) attribute the further decisions can be taken.
I.E. According to rules presented above, we can introduce a new attribute pidDifference that will hold the value of substraction of attrributes enteredPIN and correctPIN. So, those two rules can replaced with:

• if enteredPIN ≠ null and correctPIN ≠ null then pidDifference := enteredPIN - correctPIN;
• if pidDifference = 0 then authorized := true;
• if pidDifference ≠ 0 then authorized := false;