public final O add(Collection<? extends E> elements) { elements.forEach(this::add); return self(); }
@SuppressWarnings("unchecked") public final O add(E... elements) { return add(Arrays.asList(elements)); }
public final O add(E element) { elements.add(element); return self(); }
public final O remove(E element) { elements.remove(element); return self(); }
public final O add(Optional<? extends E> elementOpt) { elementOpt.ifPresent(this::add); return self(); }