Macro span 

macro_rules! span {
    () => { ... };
    (@current     $(, $($tt:tt)* )?) => { ... };
    ($broker:expr $(, $($tt:tt)* )?) => { ... };
    (@field $fields:expr, $name:ident               $(, $($tt:tt)* )?) => { ... };
    (@field $fields:expr, $name:ident = $value:expr $(, $($tt:tt)* )?) => { ... };
    (@field $fields:expr $(,)? ) => { ... };
}