Function merge 

fn merge(
    original: &[String],
    out: &mut [Option<String>],
    added: &mut Vec<String>,
    property: &str,
    wanted: &[(String, String)],
) -> bool

One property of several values: an unchanged value keeps its line; a new label rewrites only that line (keeping its group and PREF); new values are added; values gone are removed, with the lines their group carried (Apple’s item1.X-ABLabel). Returns whether anything changed.