let rec fib_rec n =
if n < 2 then
n
else
fib_rec (n - 1) + fib_rec (n - 2)
let rec fib = function
0 -> 0
| 1 -> 1
| n -> if n > 0 then fib (n-1) + fib (n-2)
else fib (n+2) - fib (n+1)