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)