fib2(0,0). % N=0, F=0 (as per definition)
fib2(1,1). % N=0, F=1 (as per definition)
fib2(N, F) :- N > 1, fib2(N, 1, 0, F). % Call fib2, N=1 => F=1, N=0 => F=0.
fib2(2, F1, F2, F) :- F is F1 + F2. % At N=2, F = F1+F2
fib2(N, F1, F2, F) :- N < 1, F is F1;
                      F3 is F1 + F2, % Calculate this Fibonacci
              N1 is N - 1, % Calculate Previous Fibonacci
              fib2(N1,F3,F1,F). %