seasons.pl

Here a solution without using negation, instead the universe of seasons is
specified.

<!-- language-all: prolog -->

    season(summer).
    season(winter).

    now(winter).

    warm(S) :-
        season(S),
        S = summer.

Some example queries:

    ?- now(S).
    S = winter ;
    false.

    ?- now(S), warm(S).
    false.

    ?- warm(S).
    S = summer ;
    false.