let rec parse args =
    match args with
    | "--help" :: xs
    | "-?" :: xs
    | "-h" :: xs ->
        printfn "Usage: ayane [options] files"
        printfn ""
        printfn "-h       Show help"
        printfn "-V       Show version"
        printfn "-v       Verbose output"
        printfn "-x lang  Input language"
        printfn "         clr"
        printfn "         dimacs"
        printfn "         tptp"
        parse xs
    | "--lang" :: xs
    | "--language" :: xs
    | "-x" :: xs ->
        match xs with
        | "clr" :: xs'
        | "dimacs" :: xs'
        | "tptp" :: xs' ->
            lang <- xs.Head
            parse xs'
        | x :: xs' ->
            failwithf "%s: unknown language" xs.Head
        | _ ->
            failwithf "%s: expected language" args.Head
    // etc