Function open_choice 

pub fn open_choice(
    at: PathPat,
    title: &str,
    icon: Icon,
    values: &[(&str, &str)],
) -> FieldRule

An offered-but-not-enforced pick-list — for a vocabulary this crate cannot see the whole of (see metadata.format), or one where a value it does not know is still legitimate (see views.*.group).