pub trait PrettyNested {
    // Required method
    fn fmt_nested(&self, cx: &PrettyCx<'_, '_>) -> Result<NestedString, Error>;
    // Provided method
    fn nested_string(&self, cx: &PrettyCx<'_, '_>) -> String { ... }
}pub trait PrettyNested {
    // Required method
    fn fmt_nested(&self, cx: &PrettyCx<'_, '_>) -> Result<NestedString, Error>;
    // Provided method
    fn nested_string(&self, cx: &PrettyCx<'_, '_>) -> String { ... }
}