mymember( X , [X | _ ] ).
mymember( X , [ _ | Z ] ) :–mymember( X , Z ).

% equivalently:  However swipl will give a warning
%      Singleton variables : Y W

mymember( X , [X | Y] ).
mymember( X , [W | Z ] ) :– mymember( X , Z ).


1?–mymember(a, [b, c, 6] ).
no
2? – mymember(a, [b, a, 6] ).
true
3? – mymember( X , [b, c, 6] ).
X = b;
X = c;
X = 6;
false





intersect( [ ], X, [ ] ).
intersect( [ X|R ], Y, [ X|Z ] ) :–
    mymember( X, Y ),   !,  intersect( R, Y, Z ).
intersect( [ X|R ], Y, Z ) :–   intersect( R, Y, Z ).


? – intersect( [ 0, 1, 6, 3 ], [ 5, 1, 8, 2, 3, 9 ], X ).
X = [ 1, 3 ]
? – intersect ( [ 1 ], [ 4 ],  X ).
X = [ ]









union( [ ],  B,  B ).
union( [H|R], B, U ) :–
 mymember( H , B ), !,  union(R, B, U ).
union([H|R], B, [H|U ] ):– union(R, B, U ).


Note: A precondition is B is a list.
The following query succeeds.
   ?- union([],a,U).
but we don't consider "a" a set since it isn't a list. Therefore we test that B is a list.

union( [ ], B, B) :- is_list(B).

? – union( [3, 7, 1, 6],  [5, 7, 2, 4, 1], X ).
X = [3, 6, 5, 7, 2, 4, 1]
/*or some other order of elements if rules are not written in the same order*/
? – union( [4], [1, 6, 8], X ).
X = [4, 1, 6, 8]