Function key_selector 

pub fn key_selector<'a>(
    layout: &Layout,
    axis: usize,
    names: impl IntoIterator<Item = &'a str>,
    within: Option<Layout>,
) -> Result<Selector, Error>

The selector that keys give on axis of layout, by key_position: one key’s position, which removes the axis, where within is None, and otherwise an index array of the keys’ positions, laid out as within.