edge(aquila, sam).
edge(sam, jeff).
edge(jessica, mariana).
edge(jeff, jeff).
edge(kuber, jeff).
edge(wei, sam).
edge(fanping, jessica).
edge(mariana, mariana).

path(X,Y) :- path(X,Y,[]).
path(X,Y,_) :- edge(X,Y).
path(X,Y,V) :- \+ member(X, V), edge(X,Z), path(Z,Y, [X|V]).