Macro lexer_builder 

macro_rules! lexer_builder {
    (
        DefaultSystem {
            number: $number:literal,
            symbol: $symbol:literal,
            keyword: $keyword:literal,
            string: $string:literal,
            comment: $comment:literal,
            whitespace: {
                allow_them: $allow_whitespace:literal,
                use_system: $whitespace:literal$(,)?
            }$(,)?
        },
        Symbols {
            Single {
                $($sym:literal => $variant:ident),* $(,)?
            }, Either {
                $($sym2:literal => $sym3:literal => $variant1:ident, $variant2:ident ),* $(,)?
            }
        },
        Keyword {
            $($x:literal => $name:ident),* $(,)?
        },
        Number {
            trailing {
                $($trail_name:literal => $trail_type:ty => $trail_enum:ident),+ $(,)?
            },
            float: $float:literal,
            u_int: $u_int:literal,
            int: $int:literal $(,)?
        }$(,)?
    ) => { ... };
}

To be done