A {@link SortedMap} extended with navigation methods returning the closest matches for given search targets. Methods {@code lowerEntry}, {@code floorEntry}, {@code ceilingEntry}, and {@code higherEntry} return {@code MapEntry} objects associated with keys respectively less than, less than or equal, greater than or equal, and greater than a given key, returning {@code null} if there is no such key. Similarly, methods {@code lowerKey}, {@code floorKey}, {@code ceilingKey}, and {@code higherKey} return only the associated keys. All of these methods are designed for locating, not traversing entries.