*
 * <pre>{@code
 *        h
 *      / | \
 *     /  e  \
 *    d       g
 *   /|\      |
 *  / | \     f
 * a  b  c
 * }</pre>
 *
 * <p>can be iterated over in preorder (hdabcegf), postorder (abcdefgh), or breadth-first order
 * (hdegabcf).
 *
 * <p>Null nodes are strictly forbidden.
 *
 * <p>Because this is an abstract class, not an interface, you can't use a lambda expression to
 * implement it:
 *

    //     the smaller of these blocks with the far end of the other one. That leaves us with a
    //     smaller version of the same problem.
    //     Say we are rotating abcdefgh by 5. We start with abcde|fgh. The smaller block is [fgh]:
    //     [abc]de|[fgh] -> [fgh]de|[abc]. Now [fgh] is in the right place, but we need to swap [de]