Macro zstack 

macro_rules! zstack {
    (alignment: $alignment:expr; $($child:expr),* $(,)?) => { ... };
    ($($child:expr),* $(,)?) => { ... };
}

Build a ZStack from a list of children, back to front.

ZStack(alignment: .topLeading) { … } is zstack![alignment: Alignment::TopLeading; …].