type t =  | Ascii of char | Code of int | Ctrl of int | Fn of int  | Backspace | Delete | Escape | Insert | Enter  | Up | Down | Left | Right | Next | Prior | Home | End let to_string = function  | Ascii c -> Printf.sprintf "'%c'" c  | Code c -> string_of_int c  | Ctrl c -> Printf.sprintf "ctrl(%#x)" c  | Fn n -> "F" ^ string_of_int n  | Backspace -> "backspace"  | Delete -> "delete"  | Escape -> "escape"  | Insert -> "insert"  | Enter -> "enter"  | Up -> "up"  | Down -> "down"  | Left -> "left"  | Right -> "right"  | Next -> "next"  | Prior -> "prior"  | Home -> "home"  | End -> "end"