pub(crate) trait Walkable<'a, V: Visitor<'a>> {
    // Required method
    fn walk_ref(&'a self, visitor: &mut V) -> V::Result;
}pub(crate) trait Walkable<'a, V: Visitor<'a>> {
    // Required method
    fn walk_ref(&'a self, visitor: &mut V) -> V::Result;
}