type t
val round : ?dir:[ `Zero | `Nearest | `Up | `Down ] -> t -> to_multiple_of:t -> tval round_towards_zero : t -> to_multiple_of:t -> tval round_down : t -> to_multiple_of:t -> tval round_up : t -> to_multiple_of:t -> tval round_nearest : t -> to_multiple_of:t -> t