steals(PERP, STUFF) :-
    thief(PERP),
    valuable(STUFF),
    owns(VICTIM,STUFF),
    not(knows(PERP,VICTIM)).

thief(badguy).
valuable(gold).
valuable(rubies).
owns(merlyn,gold).
owns(ovid,rubies).
knows(badguy,merlyn).

##</code><code>##

#!/usr/local/bin/perl
use strict;
use warnings;

use Language::Prolog::Yaswi qw/:query :assert/;
use Language::Prolog::Types::overload;
use Language::Prolog::Sugar
    functors => [qw/steals thief valuable owns knows/],    
    functors => { NOT => 'not' }, # BUGFIX: this was in chains
    vars     => [qw/PERP STUFF VICTIM/],
    chains   => {
        AND => ',',
    };

swi_assert(
    steals(PERP, STUFF) =>
        AND(
            valuable(STUFF),
            thief(PERP),
            owns(VICTIM, STUFF),
            NOT(knows(PERP, VICTIM)),
        ));

swi_facts(
    thief('badguy'),
    valuable('gold'),
    valuable('rubies'),
    owns(qw/merlyn gold/),
    owns(qw/ovid rubies/),
    knows(qw/badguy merlyn/),
);

swi_set_query(steals('badguy', STUFF));
while(swi_next) {
    print swi_var(STUFF), $/;
}