What is the logical 'not' in Prolog?

Question

The problem that I face, is a bit trivial. I want to use logical not in Prolog, but it seems that not/1 is not the thing that I want:

course(ai).
course(pl).
course(os).

have(X,Y) :- course(X),course(Y),not(X = Y).

I query:

have(X,Y), write(X-Y), nl , fail.

And I do not get the result I want :(

Answer 1

In place of not(X = Y) you need to write + X = Y or X = Y. But consider to use dif(X,Y) instead. dif/2 is present in B, SWI, YAP, SICStus. To see the difference:

?- X = b, dif(a, X).
X = b.

?- X = b, + a = X.
X = b.

So up to now everything seems to be fine. But what, if we simply exchange the order of the two goals?

?- + a = X, X = b.
false.

?- dif(a, X), X = b.
X = b.

(+)/1 now gives us a different result, because there is an answer for a = X, the goal + a = X will fail.

(+)/1 is thus not negation, but means not provable at this point in time.

A safe approximation of dif/2 is possible in ISO Prolog, too.