Macro impl_visitable 

macro_rules! impl_visitable {
    (|&mut $self:ident: $SelfTy:ty, $vis:ident: &mut $vis_ty:ident| $block:block) => { ... };
    (|&mut $self:ident: $SelfTy:ty, $vis:ident: &mut $vis_ty:ident,
      $extra:ident: $ExtraTy:ty| $block:block) => { ... };
}