Function ledger_after 

pub fn ledger_after(
    target: &Snapshot,
    live: &Snapshot,
    plan: &Plan,
    failed: &[String],
) -> Snapshot

The ledger once plan has been carried out, failed naming what could not be put back after all.

A restored package gets the snapshot’s entry, because its per-file prior records what the user had before this installer first wrote there; a fresh install would record absent and a later uninstall would delete the user’s file. A package left alone keeps its live entry, which describes what is actually on disk.