proc fold[T](self: ParsedArg; onUnknown: NimNode -> T;
             onPartial: PartialParam -> T): T

func partialArg(param: PartialParam): ParsedArg {....raises: [], tags: [],
    forbids: [].}

func unknownArg(ast: NimNode): ParsedArg {....raises: [], tags: [], forbids: [].}