val let+ : 'a option -> ('a -> 'b) -> 'b option
val and+ : 'a option -> 'b option -> ('c * 'd) option
val let* : 'a option -> ('a -> 'b option) -> 'b option
val and* : 'a option -> 'b option -> ('a * 'b) option
val (>>=) : 'a option -> ('a -> 'b option) -> 'b option