Prolog union for A U B U C

Question

I\'ve started to learn Prolog recently and I can\'t solve how to make union of three lists.

I was able to make union of 2 lists :

%element
element(X,         


        

Answer 1

You can make the union of the first two lists and then the union between that result and the third:

union(L1, L2, L3, U):-union(L1, L2, U12), union(U12, L3, U).

You can improve union/3 with a cut operator:

union([],M,M).
union([X|Y],L,S) :- element(X,L), !, union(Y,L,S).
union([X|Y],L,[X|S]) :- union(Y,L,S).