static Map<SerializedField, Object> fieldsDelta(final Map<SerializedField, Object> base,
final Map<SerializedField, Object> child) {
final Map<SerializedField, Object> m = new EnumMap<SerializedField, Object>(child);
for (final Map.Entry<SerializedField, Object> baseEntry : base.entrySet()) {
final SerializedField f = baseEntry.getKey();
if (m.containsKey(f)
&& ConfigImplUtil.equalsHandlingNull(baseEntry.getValue(), m.get(f))) {
// if field is unchanged, just remove it so we inherit
m.remove(f);
} else if (!m.containsKey(f)) {