Macro unit_argument_parser 

macro_rules! unit_argument_parser {
    (
        $parser:ident,
        $key:literal,
        $value:ty,
        parse |$reader:ident, $source:ident| $parse:block,
        suggest |$context:ident, $builder:ident| $suggest:block,
        protocol $protocol:expr
    ) => { ... };
}