f1  plane(to, ny).
f2  plane(ny, london).
f3  plane(london, bombay).
f4  plane(london, oslo).
f5  plane(bombay, katmandu).
f6  boat(oslo, stockholm).
f7  boat(stockholm, bombay).
f8  boat(bombay, maldives).

f9  leg(X,Y) :- plane(X,Y).
f10 leg(X,Y) :- boat(X,Y).

f11 trip(X,Y,[X,Y]) :- leg(X,Y).
f12 trip(X,Y,[X|Rest]) :- leg(X,Z), trip(Z,Y,Rest).





------------------------------------------------------------------------

?- [trip].
% trip compiled 0.00 sec, 2,532 bytes

Yes
?- trace, trip(to,oslo,Trip).
1  Call: (8) trip(to, oslo, _G159) ? creep
2  Call: (9) leg(to, oslo) ? creep
3  Call: (10) plane(to, oslo) ? creep
4  Fail: (10) plane(to, oslo) ? creep
5  Redo: (9) leg(to, oslo) ? creep
6  Call: (10) boat(to, oslo) ? creep
7  Fail: (10) boat(to, oslo) ? creep
8  Fail: (9) leg(to, oslo) ? creep
9  Redo: (8) trip(to, oslo, _G159) ? creep
10 Call: (9) leg(to, _G235) ? creep
11 Call: (10) plane(to, _G235) ? creep
12 Exit: (10) plane(to, ny) ? creep
13 Exit: (9) leg(to, ny) ? creep
14 Call: (9) trip(ny, oslo, _G228) ? creep
15 Call: (10) leg(ny, oslo) ? creep
16 Call: (11) plane(ny, oslo) ? creep
17 Fail: (11) plane(ny, oslo) ? creep
18 Redo: (10) leg(ny, oslo) ? creep
19 Call: (11) boat(ny, oslo) ? creep
20 Fail: (11) boat(ny, oslo) ? creep
21 Fail: (10) leg(ny, oslo) ? creep
22 Redo: (9) trip(ny, oslo, _G228) ? skip
23 Exit: (9) trip(ny, oslo, [ny, london, oslo]) ? creep
24 Exit: (8) trip(to, oslo, [to, ny, london, oslo]) ? creep

Trip = [to, ny, london, oslo]

Yes