Function group_by_family 

pub fn group_by_family(preferred: &str, formats: &[&str]) -> FamilyGrouping

Splits the files of a revision (their stored format ids) for a preferred family ("" or an unknown id = show all). Only when at least one file belongs to the preferred family are the native files of other families collapsed; neutral files and files of the preferred family stay visible. Without a file of the preferred family nothing is collapsed. Display only: nothing is removed from the data.