xform(a) :-
    X = 3,
    \+ member(X, [1,2]). % true

mforx(a) :-
    \+ member(X, [1,2]), % false
    X = 3.

vertex(a).
vertex(b).
vertex(c).
vertex(d).
vertex(e).

edge(a,b).
edge(b,d).
edge(b,c).
edge(d,c).
%
edge(c,f). % extra knoop voor testen
edge(a,g). % extra knoop voor testen

buur(X, Y) :-
    edge(X,Y)
    ;
    edge(Y,X).

/*
pad(X,Y) :-
    buur(X,Y).
pad(X, Y) :-
    buur(X, Z),
    pad(Z, Y).
    */
pad(X, Y) :-
    pad_list(X, Y, [X]).

pad_list(X, Y, List) :-
    buur(X, Y).

pad_list(X, Y, List) :-
    buur(X, Z),
    \+ member(Z, List),
    pad_list(Z, Y, [Z | List]).