fib 0 0 fib s 0 s 0 fib s s x + fib s x fib x + x 0 x + x s y s + x y 2.2 fib s s x + fib s x fib x fib s s x fib s x + x s y + x y fib s s x fib x true fib s s x fib x fib s s x fib s x true s 1 1 1 fib 1 0 0 0 0 + 2 0 fib 1 1 0 + 2 0 fib s s x + fib s x fib x false + x s y + x y true s 1 1 1 fib 1 0 0 0 0 + 2 0 fib 1 0 + 2 2 0 NaTT certifiable-1.6