steals(PERP, STUFF) :- 
             ....
      owns(VICTIM,STUFF),
      not(knows(PERP,VICTIM)).
knows(badguy,merlyn).
owns(merlyn,gold).

##</code><code>##

?- steals(badguy,X).

X = rubies

Yes

##</code><code>##

thief(merlyn).
?- steals(merlyn,X).

Yes