let out_option ?default ?(metavar = "<dot|gml|cnf|dimacs|cudf|table>") () =
let corce = function
|("cnf"|"dimacs"|"cudf"|"dot"|"gml"|"table") as s -> s
| _ -> raise Format
in
let error _ s = Printf.sprintf "%s format not supported" s in
Opt.value_option metavar default corce error