Macro fed::fed2

macro_rules! fed2 {
    ($a:ty, $b:ty) => { ... };
    ($a:ty, $b:ty; @without_children) => { ... };
    ($newtype:ty; $a:ty, $b:ty) => { ... };
    ($newtype:ty; generic: $generic:ident; $($component:ty),*; $a:ty, $b:ty) => { ... };
}