[question] How to use a SelectorVisitor from a Visitor

I'm using a Visitor to visit selectors. When I find one, I then want to use a SelectorVisitor to inspect the Selector further. I have no idea what I can provide as a suitable Impl type in the SelectorVisitor.

impl<'a> Visitor<'a> for Foo {
    type Error = ...;

    fn visit_types(&self) -> VisitTypes {
        visit_types!(SELECTORS)
    }

    fn visit_selector(&mut self, selector: &mut Selector<'a>) -> Result<(), Self::Error> {
        // Would like to know what to put in place of ??? here:
        selector.visit(???);
    }
}

and / or here:

impl<'a> SelectorVisitor<'a> for Foo {
    type Impl = ???;
    ...
}

Start by reading the Visitor, SelectorVisitor, and Selector::visit definitions to understand how the Impl type is chosen and how nested selector traversal is intended to work. Check existing implementations for a suitable pattern, then document the required usage or clarify the API so this example has an unambiguous answer.