Either

`sig
type ('a, 'b) t = Left of 'a | Right of 'b
val left : 'a -> ('a, 'b) Either.t
val right : 'b -> ('a, 'b) Either.t
val is_left : ('a, 'b) Either.t -> bool
val is_right : ('a, 'b) Either.t -> bool
val find_left : ('a, 'b) Either.t -> 'a option
val find_right : ('a, 'b) Either.t -> 'b option
val map_left : ('a1 -> 'a2) -> ('a1, 'b) Either.t -> ('a2, 'b) Either.t
val map_right : ('b1 -> 'b2) -> ('a, 'b1) Either.t -> ('a, 'b2) Either.t
val map :
left:('a1 -> 'a2) ->
right:('b1 -> 'b2) -> ('a1, 'b1) Either.t -> ('a2, 'b2) Either.t
val fold : left:('a -> 'c) -> right:('b -> 'c) -> ('a, 'b) Either.t -> 'c
val iter :
left:('a -> unit) -> right:('b -> unit) -> ('a, 'b) Either.t -> unit
val for_all :
left:('a -> bool) -> right:('b -> bool) -> ('a, 'b) Either.t -> bool
val equal :
left:('a -> 'a -> bool) ->
right:('b -> 'b -> bool) ->
('a, 'b) Either.t -> ('a, 'b) Either.t -> bool
val compare :
left:('a -> 'a -> int) ->
right:('b -> 'b -> int) -> ('a, 'b) Either.t -> ('a, 'b) Either.t -> int
end`