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>##

?- steals(badguy,X).

X = gold

Yes
?- steals(merlyn,X).

No