pub trait HasChainableValues<F: Func + ?Sized>: HasValues<F> {
// Required methods
fn values_chain<'a>(&'a self) -> Box<dyn Iterator<Item = F::Value> + 'a>;
fn values_chain_mut<'a>(
&'a mut self,
) -> Box<dyn Iterator<Item = &'a mut F::Value> + 'a>
where F: 'a;
}