Macro seq

macro_rules! seq {
    (@elem($iter:ident) .. $e:expr) => { ... };
    (@elem($iter:ident) .. $e:expr, $($t:tt)*) => { ... };
    (@elem($iter:ident) $e:expr, $($t:tt)*) => { ... };
    (@elem($iter:ident)) => { ... };
    (@elem($iter:ident) $($t:tt)*) => { ... };
    () => { ... };
    ($($t:tt)*) => { ... };
}

Replacement for the vec macro that supports ..foo notation to flatten vectors.