?- pos(X, Y) = pos(1,2).
X = 1,
Y = 2.

?- X = 1+2.
X = 1+2.

?- X = 1+2, Y is X.
X = 1+2,
Y = 3.

?- X+Y+Z = 1+2+[1,2,3].
X = 1,
Y = 2,
Z = [1, 2, 3].


?- a @< b.
true.

?- 1 @< b.
true.

?- b @< 1.
false.

?- b @< [1,2].
true.

https://www.swi-prolog.org/pldoc/man?section=standardorder