How to program set(X) + set(a) = set(X)

Hello evrtybody,

I'm going to code in prolog the 'set union' which takes into

consideration set(X) which the set X is undetermined.

The set union is like this:

set(X) + set(a) =set(X).

If it works, it might be used for any set operation such as subtract,

implication, etc..

Do you have on idea of the implementation?

Thank you.