let orec fib x = match x with
 | 0 -> 0
 | 1 -> 1
 | n -> fib (n - 1) + fib (n - 2)

let rec f = function
  |1 -> 0
  |n -> f (n-1)
;;

let orec f = function
  |1 -> 0
  |n -> f (n-1)
;;

let orec f = function
  | 1 -> 0
  | n when (n mod 2) = 0 -> f (n - 1)
  | n -> g ( n - 2 )
and g = function
  | 0 -> 0
  | n when (n mod 2) > 0 -> g ( n - 1 )
  | n -> f ( n - 1 )
;;