true.

?- likes(eve, What); person(What).
What = pie ;;
What = tom ;;
What = eve ;;
What = tom.

?- likes(eve, What), \+(likes(al, What)).
What = pie ;;
What = tom ;;
false.

?- likes(eve, What), \+(likes(al, What)).
What = pie 
.

?- halt.