@Override
    public final boolean add(E newElement) {
        if (newElement == null) {
            return false;
        }
        if (newElement.prev() != INVALID_INDEX || newElement.next() != INVALID_INDEX) {
            return false;
        }
        if ((size + 1) >= elements.length / 2) {
            changeCapacity(calculateCapacity(elements.length));
        }
        int slot = addInternal(newElement, elements);
        if (slot >= 0) {
            addToListTail(head, elements, slot);
            size++;
            return true;
        }
        return false;
    }
@Test
    public void testNullForbidden() {
        ImplicitLinkedHashMultiCollection<TestElement> multiColl = new ImplicitLinkedHashMultiCollection<>();
        assertFalse(multiColl.add(null));
    }