sig
val is_some : 'a option -> bool
val is_none : 'a option -> bool
val value : 'a option -> 'a
val default : 'a -> 'a option -> 'a
val classify : ('a -> bool) -> 'a -> 'a option
val perform : ('a -> unit) -> 'a option -> unit
val map : ('a -> 'b) -> 'a option -> 'b option
val filter : ('a -> bool) -> 'a option -> 'a option
val flatten : 'a option option -> 'a option
end