I'm having some difficulties in prolog, I'm trying to write a predicate that will return all paths between two cities, although at the moment it returns the first path it finds on an infinite loop. Not sure where I'm going wrong but I've been trying to figure this out all day and I'm getting nowhere.

Any help that could be offered would be appreciated.

go:-
    repeat,
    f([],0,lon,spa,OP,OD),
    write(OP),
    write(OD),
    fail.

city(lon).
city(ath).
city(spa).
city(kol).

path(lon,1,ath).
path(ath,3,spa).
path(spa,2,kol).
path(lon,1,kol).

joined(X,Y,D):-
    path(X,D,Y);path(Y,D,X).

f(Ci_Vi,Di,De,De,PaO,Di):-
    append([De],Ci_Vi,PaO),
    !.
f(Cities_Visited,Distance,Start,Destination,Output_Path,Output_Distance):-
    repeat,
    city(X),
    joined(Start,X,D),
    not_member(X,Cities_Visited),
    New_Distance is Distance + D,
    f([Start|Cities_Visited],New_Distance,X,Destination,Output_Path,Output_Distance).

not_member(X,List):-
    member(X,List),
    !,
    fail.
not_member(X,List).

The output I'm expecting here is [spa,ath,lon]4 [spa,kol,lon]3.

Once again, any help would be appreciated.

Many thanks in advance.