Module Either

module Either: sig .. end

type ('a, 'b) either = 
| Left of 'a
| Right of 'b
val is_left : ('a, 'b) either -> bool
val is_right : ('a, 'b) either -> bool
val assert_left : ('a, 'b) either -> 'a
val assert_right : ('a, 'b) either -> 'b