Macro nested

macro_rules! nested {
    ($($i:ident in $iter:expr)=>* => $body:block ) => { ... };
    (@loop $body:block, $i:ident in $iter:expr) => { ... };
    (@loop $body:block, $i:ident in $iter:expr) => { ... };
    (@loop $body:block, $i:ident in $iter:expr, $($tail:tt)*) => { ... };
}