pub trait SetIndex<S: Set> {
// Required methods
fn ref_from<'a>(
&self,
set: &'a S,
) -> Result<&'a <S::Ctor as Ctor>::Opt, Error>;
fn mut_from<'a>(
&self,
set: &'a mut S,
) -> Result<&'a mut <S::Ctor as Ctor>::Opt, Error>;
}