*       UnsupportedOperationException when those mutators are called.
   * </ol>
   *
   * @param collection the presumed-immutable collection
   * @param sampleElement an element of the same type as that contained by {@code collection}.
   *     {@code collection} may or may not have {@code sampleElement} as a member.
   */
  public static <E extends @Nullable Object> void assertCollectionIsUnmodifiable(

    return delegate.floor(checkValid(e));
  }

  @Override
  public SortedSet<E> headSet(E toElement) {
    return headSet(toElement, false);
  }

  @Override
  public NavigableSet<E> headSet(E toElement, boolean inclusive) {
    return new SafeTreeSet<>(delegate.headSet(checkValid(toElement), inclusive));
  }

  @Override
  public @Nullable E higher(E e) {

  /** Describes how a list is modifiable */
  public enum Modifiability {
    NONE, // immutable lists
    BY_ELEMENT, // elements can change (set), but not structure
    DIRECT_ONLY, // Element can be added and removed only via direct calls, not through iterators
    ALL // Elements can be added and removed as well as modified.
  }

  /** Handles the creation of lists needed for the tests */

    this.equivalence = checkNotNull(equivalence);
  }

  /**
   * Different orderings for combining the elements of an input array, which must all produce the
   * same result.
   */
  enum CollectStrategy {
    /** Get one accumulator and accumulate the elements into it sequentially. */
    SEQUENTIAL {
      @Override

  }

  /**
   * Adds the given element to this queue. If this queue has a maximum size, after adding {@code
   * element} the queue will automatically evict its greatest element (according to its comparator),
   * which may be {@code element} itself.
   *
   * @return {@code true} always
   */
  @CanIgnoreReturnValue
  @Override
  public boolean add(E element) {
    offer(element);
    return true;
  }

   * of their elements."
   *
   * </blockquote>
   */
  RESTRICTS_ELEMENTS,

  /**
   * Indicates that a collection has a well-defined ordering of its elements. The ordering may
   * depend on the element values, such as a {@link SortedSet}, or on the insertion ordering, such
   * as a {@link LinkedHashSet}. All list tests and sorted-collection tests automatically specify
   * this feature.
   */

  public SampleElements<Object> samples() {
    return new Colliders();
  }

  @Override
  public Object[] createArray(int length) {
    return new Object[length];
  }

  /** Returns the original element list, unchanged. */
  @Override
  public List<Object> order(List<Object> insertionOrder) {
    return insertionOrder;
  }

        if (newLength > elements.length) {
          elements = Arrays.copyOf(elements, newLength);
        }
      }
      elements[n++] = element;
      return this;
    }

    /**
     * Adds each element of {@code elements} to the {@code ImmutableSortedSet}, ignoring duplicate
     * elements (only the first duplicate element is added).
     *
     * @param elements the elements to add

     * elements (only the first duplicate element is added).
     *
     * @param elements the elements to add
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code elements} is null or contains a null element
     */
    @Override
    @CanIgnoreReturnValue
    public Builder<E> addAll(Iterable<? extends E> elements) {

  // This reference is used both by the custom field serializer, and by the
  // GWT compiler to infer the elements of the lists that needs to be
  // serialized.
  //
  // Although this reference is non-final, it doesn't change after set creation.
  E element;

  SingletonImmutableSet(E element) {
    this.element = checkNotNull(element);
  }

  @Override
  public int size() {
    return 1;
  }

  @Override