*
   * - Define putIfAbsent and replace as treating zero and absent identically (as currently
   *   implemented below). This is a bit surprising with putIfAbsent, which really becomes
   *   putIfZero.
   *
   * - Allow putIfAbsent and replace to distinguish between zero and absent, but don't implement
   *   remove(K, long). Without any two-phase operations it becomes feasible for all remaining

 * </ul>
 *
 * <h3>Warnings</h3>
 *
 * <ul>
 *   <li>Use immutable value types only, if at all possible. If you must use a mutable type, <b>do
 *       not</b> allow the endpoint instances to mutate after the range is created!
 *   <li>Your value type's comparison method should be {@linkplain Comparable consistent with