Function plan 

pub fn plan(
    target: &Snapshot,
    live: &Snapshot,
    payload_present: &dyn Fn(&str) -> bool,
) -> Plan

What it takes to make live look like target.

payload_present says whether a digest is in the cache, so the decision is testable without one. An empty target is a complete uninstall: the same operation, deliberately not a second code path.