?- consult('combosets.pro').
% combosets.pro compiled 0.01 sec, 7,384 bytes

Yes
?- combos(set(X,Y,Z), combo(X,Y),extra(Z)).

Yes
?- combos(set(1,2,3), COM,EX).

COM = combo(1, 2)
EX = extra(3)

Yes
?- combos(set(1,2,3), COM,EX).

COM = combo(1, 2)
EX = extra(3) ;

COM = combo(2, 3)
EX = extra(1) ;

COM = combo(1, 3)
EX = extra(2) ;

No
?- combos(set(a,b,c,d),Com,EX).

Com = combo(a, b)
EX = extras(c, d) ;

Com = combo(a, c)
EX = extras(b, d) ;

Com = combo(a, d)
EX = extras(b, c) ;

Com = combo(b, c)
EX = extras(a, d) ;

Com = combo(b, d)
EX = extras(a, c) ;

Com = combo(c, d)
EX = extras(a, b) ;

No
?- combos(set(1,2,3,a,b),COM,EX).

COM = combo(1, 2)
EX = extras(3, a, b) ;

COM = combo(1, 3)
EX = extras(2, a, b) ;

COM = combo(1, a)
EX = extras(2, 3, b) ;

COM = combo(1, b)
EX = extras(2, 3, a) ;

COM = combo(2, 3)
EX = extras(1, a, b) ;

COM = combo(2, a)
EX = extras(1, 3, b) ;

COM = combo(2, b)
EX = extras(1, 3, a) ;

COM = combo(3, a)
EX = extras(1, 2, b) ;

COM = combo(3, b)
EX = extras(1, 2, a) ;

COM = combo(a, b)
EX = extras(1, 2, 3) ;

No
?- halt.
moxie - swenton - A2 >