public Set<String> oldParents() {
            Set<String> oldParents = new HashSet<>();
            for (var entry : historyEntries)
                entry.tenant.parentHostname().ifPresent(oldParents::add);
            return oldParents;
        }
Provides convenient methods for tallying failures.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/maintenance/CapacityChecker.java