public static Thing updateA( final String uid, final int value ) {

    return ofy().transact(new Work<Thing>() {
        @Override
        public Thing run() {
            Thing thing = ofy().load().type(Thing.class).id(uid).safe();
            if (thing.a != value) {
                thing.a = value;
                ofy().save().entity(thing);
            } else {
        // there is no save() required here and I get away with 1 operation cheaper, yet all is serial and safe?
            }
            return thing;
        }
    });
}