true.

?- likes(A, pie).
A = eve ;;
false.

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

?- likes(A, eve), A == al.
A = al ;;
false.

?- likes(al, eve).
true.

?- likes(What, eve).
What = al ;;
What = eve.

?- likes(D, D).
D = eve.

?- likes(eve, Who), person(Who).
Who = tom ;;
false.

?- halt.