Function publish_closed 

pub fn publish_closed(
    store: &Path,
    unit: &str,
    tip: &str,
    selection: &Selection,
    tmp: &Path,
) -> Result<Option<String>, String>

Publishes selection as one commit under refs/speccoder/held-out/<unit> of the private store (publication_store): <key>/manifest.json and <key>/files/… of every set, parented on tip (the pushed branch) and on the ref’s previous commit, so the ref only grows and every push of it is a fast-forward. Ok(None) when nothing is selected.