Macro arg 

macro_rules! arg {
    ($x:ident; (var($e:ident))) => { ... };
    ($x:ident; ($f:tt $e:tt?)) => { ... };
    ($x:ident; ($e:tt?)) => { ... };
    ($x:ident; ($e:tt..)) => { ... };
    ($x:ident; $e:tt) => { ... };
}