equal(X, X).
"="(andy, andy_2639).
"="(andreas, andy).

?- equal(andy_2639, X).


"="(Y, nachbar_von(X), d) :- write("X: ", X, "; Y: ", Y),
                             nachbarn(Y, X),
                             write("X: ", X, "; Y: ", Y).
nachbarn(a, b).
nachbarn(b, c).
nachbarn(X, Y) :- nachbarn(Y, X).

?- equal(nachbar_von(X), Y).