val return : 'a -> 'a option
val (>>=) : 'a option -> ('a -> 'a option) -> 'a optiont >>= f returns a computation that sequences the computations represented by two monad elements. The resulting computation first does t to yield a value v, and then runs the computation returned by f v.
val (>>|) : 'a option -> ('a -> 'b) -> 'a optiont >>| f is t >>= (fun a -> return (f a)).