How to write a Function

I want to write a function in AIMMS which should return a value or an element of Set X(say).

The function is say F(a,b,x) where a in A, b in B, and x in X. The function F should return an element(or value) x1 from set X if x1 is in X otherwise returns no value.

for Example, say F(a,b,x) = x - 10 and X = {x | 0<= x <= 100}

F(a,b,20) should return 10  and F(a,b,8) returns no value or a default value which AIMMS will not consider when using this functions return value in a constraint.