`type ('a, 'b) t = ('a, 'b) Stdlib.Either.t =`

1. `| Left of 'a`
2. `| Right of 'b`

A value of `('a, 'b) Either.t` contains either a value of `'a` or a value of `'b`

`val left : 'a -> ('a, 'b) t`

`left v` is `Left v`.

`val right : 'b -> ('a, 'b) t`

`right v` is `Right v`.

`val is_left : ('a, 'b) t -> bool`

`is_left (Left v)` is `true`, `is_left (Right v)` is `false`.

`val is_right : ('a, 'b) t -> bool`

`is_right (Left v)` is `false`, `is_right (Right v)` is `true`.

`val find_left : ('a, 'b) t -> 'a option`

`find_left (Left v)` is `Some v`, `find_left (Right _)` is `None`

`val find_right : ('a, 'b) t -> 'b option`

`find_right (Right v)` is `Some v`, `find_right (Left _)` is `None`

`val map_left : ('a1 -> 'a2) -> ('a1, 'b) t -> ('a2, 'b) t`

`map_left f e` is `Left (f v)` if `e` is `Left v` and `e` if `e` is `Right _`.

`val map_right : ('b1 -> 'b2) -> ('a, 'b1) t -> ('a, 'b2) t`

`map_right f e` is `Right (f v)` if `e` is `Right v` and `e` if `e` is `Left _`.

`val map : left:('a1 -> 'a2) -> right:('b1 -> 'b2) -> ('a1, 'b1) t -> ('a2, 'b2) t`

`map ~left ~right (Left v)` is `Left (left v)`, `map ~left ~right (Right v)` is `Right (right v)`.

`val fold : left:('a -> 'c) -> right:('b -> 'c) -> ('a, 'b) t -> 'c`

`fold ~left ~right (Left v)` is `left v`, and `fold ~left ~right (Right v)` is `right v`.

`val iter : left:('a -> unit) -> right:('b -> unit) -> ('a, 'b) t -> unit`

`iter ~left ~right (Left v)` is `left v`, and `iter ~left ~right (Right v)` is `right v`.

`val for_all : left:('a -> bool) -> right:('b -> bool) -> ('a, 'b) t -> bool`

`for_all ~left ~right (Left v)` is `left v`, and `for_all ~left ~right (Right v)` is `right v`.

`val equal : left:('a -> 'a -> bool) -> right:('b -> 'b -> bool) -> ('a, 'b) t -> ('a, 'b) t -> bool`

`equal ~left ~right e0 e1` tests equality of `e0` and `e1` using `left` and `right` to respectively compare values wrapped by `Left _` and `Right _`.

`val compare : left:('a -> 'a -> int) -> right:('b -> 'b -> int) -> ('a, 'b) t -> ('a, 'b) t -> int`

`compare ~left ~right e0 e1` totally orders `e0` and `e1` using `left` and `right` to respectively compare values wrapped by `Left _` and `Right _`. `Left _` values are smaller than `Right _` values.