macro_rules! impl_via_ambassador {
    { 
    $(
      $( #[ $attr:meta ] )*
      impl $( [ $($generics:tt)* ] )? $Trait:ident for $Type:ty
      $( where [ $($where:tt)* ] )?
      { $($how_immut:tt)* }
    )*
  } => { ... };
}