### Question:
INode extends BasicNode { static VerifyException invalidElement(final BasicNode elem) { throw new VerifyException("An INode can host only a CNode, a TNode or an LNode, not " + elem); } INode(final Gen gen, final MainNode<K, V> mainnode);   }### Answer:
@Test public void testInvalidElement() { assertThrows(VerifyException.class, () -> INode.invalidElement(null)); }