pub trait BackendExt: Backend {
    // Provided method
    fn extend<T: Backend>(self, other: T) -> CombinedBackend<Self, T>
       where Self: Sized { ... }
}pub trait BackendExt: Backend {
    // Provided method
    fn extend<T: Backend>(self, other: T) -> CombinedBackend<Self, T>
       where Self: Sized { ... }
}