Macro combinator 

macro_rules! combinator {
    (~ $($selector:tt)+) => { ... };
    (+ $($selector:tt)+) => { ... };
    (, $($selector:tt)+) => { ... };
    (> $($selector:tt)+) => { ... };
    ($head:ident $($selector:tt)+) => { ... };
    (_ $($selector:tt)+) => { ... };
    ({$($selector:tt)*}) => { ... };
    ({$($selector:tt)*} $($next:tt)+) => { ... };
    ([$($inner:tt)*] $($next:tt)+) => { ... };
    ($($rest:tt)*) => { ... };
}