fib(0,0) :- !.
fib(1,1) :- !.
fib(N,F) :-
    N > 1,
    N1 is N-1,
    N2 is N-2,
    fib(N1,F1),
    fib(N2,F2),
    F is F1+F2.

:- fib(10,F), write(F), nl.