private static <T> Map<AccessorKey, T> diff(Map<AccessorKey, T> first, Map<AccessorKey, T> second) {
        return first.entrySet().stream()
            .filter(e -> !second.containsKey(e.getKey()))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
    }

            if (errors.isEmpty()) {
                fw.println("All clear!");
                return;
            }
            for (Map.Entry<File, List<Error>> e : errors.entrySet()) {
                File file = e.getKey();
                List<Error> errorsForFile = e.getValue();

                StringBuilder sb = new StringBuilder();
                for (Error error : errorsForFile) {

        Map<AccessorKey, ReplacedAccessor> keptAccessors = new HashMap<>(oldAccessorsOfUpgradedProperties);
        keptAccessors.entrySet().removeIf(e -> {
            if (seenUpgradedAccessorsChanges.contains(e.getKey())) {
                return true;
            }
            ReplacedAccessor accessor = e.getValue();
            return accessor.getBinaryCompatibility() == BinaryCompatibility.ACCESSORS_KEPT;
        });