Macro impl_selectable 

macro_rules! impl_selectable {
    ($struct:ident) => { ... };
}

Implement the SelectableContent for struct $struc with content type $content_type. This trait allows to navigate through a vector of element content_type. It implements: is_empty, len, next, prev, set_index and selected_is_last. This macro should not be called directly. Use crate::impl_content instead.