Macro focused_keys 

macro_rules! focused_keys {
    ($(
        $(#[$meta:meta])*
        $name:ident : $ty:ty
    ),* $(,)?) => { ... };
}

Declare focused-value keys of your own — as environment_keys!, for FocusedValues.

No default: a focused value is T? and absent until something publishes it, which is what lets a menu command know whether anything is focused at all.

focused_keys! {
    /// What the focused window has selected.
    selection: String,
}

text("row").focused_scene_value(selection(), "the third row")