Function selected_files 

fn selected_files<'a, T: FileMatches + Clone>(
    files: &'a [T],
    file_type: &'a str,
    selected: &'a BTreeMap<(&str, usize), HashSet<usize>>,
) -> impl Iterator<Item = T> + 'a

Returns copies of the files of a type in open packs with selected matches, keeping only those matches.