What is the best place to implement custom function like substraction of two field values of a fact in BRMS 6 ?

Issue

• When choosing formula in modifying constraints for fact, what is the best way to implement following function?
  e.g. User needs to calculate the difference between 2 integer numbers. The condition can be written as if (a-b)>5 . Where a and b are integer fields of the same fact.
• What is the best practice to do implement it? Should users write a function for that or there are built-in sum/average/count etc. functions?