static public <T> Value<T> empty(String key) { return new Value(key, null); }
static public <T> Value<T> of(String key, T value) { return new Value(key, value); }
static public <T> Value<T> of(String key, T value) { return new Value(key, value); }
static public <T> Value<T> empty(String key) { return new Value(key, null); }