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(**

   Optional non-negative integers can be represented by an [int]
   provided that [None] is encoded as [-1] and [Some x] is encoded by
   [x]. This module provides a type abstraction to maintain this
   invariant.

   This optimized representation avoids the allocation of a block in
   the heap and also removes one indirection when reading such an
   optional non-negative integer.

   @raise [Assert_failure] when assumptions are broken. This module is meant to
   be used internally only, covered by an extensive test-suite with assertion
   checks on, and packaged with assertion checks off.

*)

type t = private int

(** An absent non-negative integer. *)
val none : t

(** [some x] assumes that [x] is non-negative. *)
val some : int -> t

(** [is_none x] returns [x] iff [x] is an absent non-negative integer. *)
val is_none : t -> bool

(** [is_some x] returns [x] iff [x] is a non-negative integer. *)
val is_some : t -> bool

(** [fold ~none:d some:f x = d] if [x] is absent and [f i] if [x = some i]. *)
val fold : none:'a -> some:(int -> 'a) -> t -> 'a

(** [get x] returns [i] assuming that [x = some i]. *)
val get : t -> int