val map : ('a -> 'b) -> 'a option -> 'b option
val fold : ('a -> 'b -> 'a) -> 'a -> 'b option -> 'a
val bind : ('a -> 'b option) -> 'a option -> 'b option
val unsome : 'a -> 'a option -> 'a
val equal : ('a -> 'a -> bool) -> 'a option -> 'a option -> bool