likes(ann,X) :- toy(X), plays(ann,X).
likes(sue,X) :- likes(ann,X).
toy(doll).
toy(snoopy).
plays(ann,snoopy).


?- trace.

?- likes(sue,X).
   Call: (7) likes(sue, _G442) ? creep
   Call: (8) likes(ann, _G442) ? creep
   Call: (9) toy(_G442) ? creep
   Exit: (9) toy(doll) ? creep
   Call: (9) plays(ann, doll) ? creep
   Fail: (9) plays(ann, doll) ? creep
   Redo: (9) toy(_G442) ? creep
   Exit: (9) toy(snoopy) ? creep
   Call: (9) plays(ann, snoopy) ? creep
   Exit: (9) plays(ann, snoopy) ? creep
   Exit: (8) likes(ann, snoopy) ? creep
   Exit: (7) likes(sue, snoopy) ? creep

X = snoopy 

Yes