Function cover 

pub fn cover<R, S, P>(open: S, build: impl Fn(&R) -> P + 'static) -> Cover<S, R>
where
    R: Route,
    S: Binding<Option<R>>,
    P: Piece,

A fullscreen cover over open: Some(r) presents build(&r), None dismisses (docs/cover.md). Registers a string-route adapter, so navigate("<key>") opens it and nav_back() closes it, and current_route() reports the presented key.