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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
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   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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   */
  
  package java.util;
  
  /**
   * A collection that contains no duplicate elements.  More formally, sets
   * contain no pair of elements {@code e1} and {@code e2} such that
   * {@code e1.equals(e2)}, and at most one null element.  As implied by
   * its name, this interface models the mathematical <i>set</i> abstraction.
   *
   * <p>The {@code Set} interface places additional stipulations, beyond those
   * inherited from the {@code Collection} interface, on the contracts of all
   * constructors and on the contracts of the {@code add}, {@code equals} and
   * {@code hashCode} methods.  Declarations for other inherited methods are
   * also included here for convenience.  (The specifications accompanying these
   * declarations have been tailored to the {@code Set} interface, but they do
   * not contain any additional stipulations.)
   *
   * <p>The additional stipulation on constructors is, not surprisingly,
   * that all constructors must create a set that contains no duplicate elements
   * (as defined above).
   *
   * <p>Note: Great care must be exercised if mutable objects are used as set
   * elements.  The behavior of a set is not specified if the value of an object
   * is changed in a manner that affects {@code equals} comparisons while the
   * object is an element in the set.  A special case of this prohibition is
   * that it is not permissible for a set to contain itself as an element.
   *
   * <p>Some set implementations have restrictions on the elements that
   * they may contain.  For example, some implementations prohibit null elements,
   * and some have restrictions on the types of their elements.  Attempting to
   * add an ineligible element throws an unchecked exception, typically
   * {@code NullPointerException} or {@code ClassCastException}.  Attempting
   * to query the presence of an ineligible element may throw an exception,
   * or it may simply return false; some implementations will exhibit the former
   * behavior and some will exhibit the latter.  More generally, attempting an
   * operation on an ineligible element whose completion would not result in
   * the insertion of an ineligible element into the set may throw an
   * exception or it may succeed, at the option of the implementation.
   * Such exceptions are marked as "optional" in the specification for this
   * interface.
   *
   * <h2><a id="immutable">Immutable Set Static Factory Methods</a></h2>
   * <p>The {@link Set#of(Object...) Set.of()} static factory methods
   * provide a convenient way to create immutable sets. The {@code Set}
   * instances created by these methods have the following characteristics:
   *
   * <ul>
   * <li>They are <em>structurally immutable</em>. Elements cannot be added or
   * removed. Calling any mutator method will always cause
   * {@code UnsupportedOperationException} to be thrown.
   * However, if the contained elements are themselves mutable, this may cause the
   * Set to behave inconsistently or its contents to appear to change.
   * <li>They disallow {@code null} elements. Attempts to create them with
   * {@code null} elements result in {@code NullPointerException}.
   * <li>They are serializable if all elements are serializable.
   * <li>They reject duplicate elements at creation time. Duplicate elements
   * passed to a static factory method result in {@code IllegalArgumentException}.
   * <li>The iteration order of set elements is unspecified and is subject to change.
   * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
   * Callers should make no assumptions about the identity of the returned instances.
   * Factories are free to create new instances or reuse existing ones. Therefore,
   * identity-sensitive operations on these instances (reference equality ({@code ==}),
   * identity hash code, and synchronization) are unreliable and should be avoided.
   * <li>They are serialized as specified on the
   * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
   * page.
   * </ul>
   *
   * <p>This interface is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @param <E> the type of elements maintained by this set
   *
   * @author  Josh Bloch
   * @author  Neal Gafter
   * @see Collection
   * @see List
   * @see SortedSet
   * @see HashSet
   * @see TreeSet
   * @see AbstractSet
   * @see Collections#singleton(Object)
   * @see Collections#EMPTY_SET
   * @since 1.2
   */
  
  public interface Set<E> extends Collection<E> {
      // Query Operations
  
      /**
       * Returns the number of elements in this set (its cardinality).  If this
       * set contains more than {@code Integer.MAX_VALUE} elements, returns
       * {@code Integer.MAX_VALUE}.
       *
       * @return the number of elements in this set (its cardinality)
       */
      int size();
  
      /**
       * Returns {@code true} if this set contains no elements.
       *
       * @return {@code true} if this set contains no elements
       */
      boolean isEmpty();
  
      /**
       * Returns {@code true} if this set contains the specified element.
       * More formally, returns {@code true} if and only if this set
       * contains an element {@code e} such that
       * {@code Objects.equals(o, e)}.
       *
       * @param o element whose presence in this set is to be tested
       * @return {@code true} if this set contains the specified element
       * @throws ClassCastException if the type of the specified element
       *         is incompatible with this set
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified element is null and this
       *         set does not permit null elements
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       */
      boolean contains(Object o);
  
      /**
       * Returns an iterator over the elements in this set.  The elements are
       * returned in no particular order (unless this set is an instance of some
       * class that provides a guarantee).
       *
       * @return an iterator over the elements in this set
       */
      Iterator<E> iterator();
  
      /**
       * Returns an array containing all of the elements in this set.
       * If this set makes any guarantees as to what order its elements
       * are returned by its iterator, this method must return the
       * elements in the same order.
       *
       * <p>The returned array will be "safe" in that no references to it
       * are maintained by this set.  (In other words, this method must
       * allocate a new array even if this set is backed by an array).
       * The caller is thus free to modify the returned array.
       *
       * <p>This method acts as bridge between array-based and collection-based
       * APIs.
       *
       * @return an array containing all the elements in this set
       */
      Object[] toArray();
  
      /**
       * Returns an array containing all of the elements in this set; the
       * runtime type of the returned array is that of the specified array.
       * If the set fits in the specified array, it is returned therein.
       * Otherwise, a new array is allocated with the runtime type of the
       * specified array and the size of this set.
       *
       * <p>If this set fits in the specified array with room to spare
       * (i.e., the array has more elements than this set), the element in
       * the array immediately following the end of the set is set to
       * {@code null}.  (This is useful in determining the length of this
       * set <i>only</i> if the caller knows that this set does not contain
       * any null elements.)
       *
       * <p>If this set makes any guarantees as to what order its elements
       * are returned by its iterator, this method must return the elements
       * in the same order.
       *
       * <p>Like the {@link #toArray()} method, this method acts as bridge between
       * array-based and collection-based APIs.  Further, this method allows
       * precise control over the runtime type of the output array, and may,
       * under certain circumstances, be used to save allocation costs.
       *
       * <p>Suppose {@code x} is a set known to contain only strings.
       * The following code can be used to dump the set into a newly allocated
       * array of {@code String}:
       *
       * <pre>
       *     String[] y = x.toArray(new String[0]);</pre>
       *
       * Note that {@code toArray(new Object[0])} is identical in function to
       * {@code toArray()}.
       *
       * @param a the array into which the elements of this set are to be
       *        stored, if it is big enough; otherwise, a new array of the same
       *        runtime type is allocated for this purpose.
       * @return an array containing all the elements in this set
       * @throws ArrayStoreException if the runtime type of the specified array
       *         is not a supertype of the runtime type of every element in this
       *         set
       * @throws NullPointerException if the specified array is null
       */
      <T> T[] toArray(T[] a);
  
  
      // Modification Operations
  
      /**
       * Adds the specified element to this set if it is not already present
       * (optional operation).  More formally, adds the specified element
       * {@code e} to this set if the set contains no element {@code e2}
       * such that
       * {@code Objects.equals(e, e2)}.
       * If this set already contains the element, the call leaves the set
       * unchanged and returns {@code false}.  In combination with the
       * restriction on constructors, this ensures that sets never contain
       * duplicate elements.
       *
       * <p>The stipulation above does not imply that sets must accept all
       * elements; sets may refuse to add any particular element, including
       * {@code null}, and throw an exception, as described in the
       * specification for {@link Collection#add Collection.add}.
       * Individual set implementations should clearly document any
       * restrictions on the elements that they may contain.
       *
       * @param e element to be added to this set
       * @return {@code true} if this set did not already contain the specified
       *         element
       * @throws UnsupportedOperationException if the {@code add} operation
       *         is not supported by this set
       * @throws ClassCastException if the class of the specified element
       *         prevents it from being added to this set
       * @throws NullPointerException if the specified element is null and this
       *         set does not permit null elements
       * @throws IllegalArgumentException if some property of the specified element
       *         prevents it from being added to this set
       */
      boolean add(E e);
  
  
      /**
       * Removes the specified element from this set if it is present
       * (optional operation).  More formally, removes an element {@code e}
       * such that
       * {@code Objects.equals(o, e)}, if
       * this set contains such an element.  Returns {@code true} if this set
       * contained the element (or equivalently, if this set changed as a
       * result of the call).  (This set will not contain the element once the
       * call returns.)
       *
       * @param o object to be removed from this set, if present
       * @return {@code true} if this set contained the specified element
       * @throws ClassCastException if the type of the specified element
       *         is incompatible with this set
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified element is null and this
       *         set does not permit null elements
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws UnsupportedOperationException if the {@code remove} operation
       *         is not supported by this set
       */
      boolean remove(Object o);
  
  
      // Bulk Operations
  
      /**
       * Returns {@code true} if this set contains all of the elements of the
       * specified collection.  If the specified collection is also a set, this
       * method returns {@code true} if it is a <i>subset</i> of this set.
       *
       * @param  c collection to be checked for containment in this set
       * @return {@code true} if this set contains all of the elements of the
       *         specified collection
       * @throws ClassCastException if the types of one or more elements
       *         in the specified collection are incompatible with this
       *         set
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified collection contains one
       *         or more null elements and this set does not permit null
       *         elements
       * (<a href="Collection.html#optional-restrictions">optional</a>),
       *         or if the specified collection is null
       * @see    #contains(Object)
       */
      boolean containsAll(Collection<?> c);
  
      /**
       * Adds all of the elements in the specified collection to this set if
       * they're not already present (optional operation).  If the specified
       * collection is also a set, the {@code addAll} operation effectively
       * modifies this set so that its value is the <i>union</i> of the two
       * sets.  The behavior of this operation is undefined if the specified
       * collection is modified while the operation is in progress.
       *
       * @param  c collection containing elements to be added to this set
       * @return {@code true} if this set changed as a result of the call
       *
       * @throws UnsupportedOperationException if the {@code addAll} operation
       *         is not supported by this set
       * @throws ClassCastException if the class of an element of the
       *         specified collection prevents it from being added to this set
       * @throws NullPointerException if the specified collection contains one
       *         or more null elements and this set does not permit null
       *         elements, or if the specified collection is null
       * @throws IllegalArgumentException if some property of an element of the
       *         specified collection prevents it from being added to this set
       * @see #add(Object)
       */
      boolean addAll(Collection<? extends E> c);
  
      /**
       * Retains only the elements in this set that are contained in the
       * specified collection (optional operation).  In other words, removes
       * from this set all of its elements that are not contained in the
       * specified collection.  If the specified collection is also a set, this
       * operation effectively modifies this set so that its value is the
       * <i>intersection</i> of the two sets.
       *
       * @param  c collection containing elements to be retained in this set
       * @return {@code true} if this set changed as a result of the call
       * @throws UnsupportedOperationException if the {@code retainAll} operation
       *         is not supported by this set
       * @throws ClassCastException if the class of an element of this set
       *         is incompatible with the specified collection
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if this set contains a null element and the
       *         specified collection does not permit null elements
       *         (<a href="Collection.html#optional-restrictions">optional</a>),
       *         or if the specified collection is null
       * @see #remove(Object)
       */
      boolean retainAll(Collection<?> c);
  
      /**
       * Removes from this set all of its elements that are contained in the
       * specified collection (optional operation).  If the specified
       * collection is also a set, this operation effectively modifies this
       * set so that its value is the <i>asymmetric set difference</i> of
       * the two sets.
       *
       * @param  c collection containing elements to be removed from this set
       * @return {@code true} if this set changed as a result of the call
       * @throws UnsupportedOperationException if the {@code removeAll} operation
       *         is not supported by this set
       * @throws ClassCastException if the class of an element of this set
       *         is incompatible with the specified collection
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if this set contains a null element and the
       *         specified collection does not permit null elements
       *         (<a href="Collection.html#optional-restrictions">optional</a>),
       *         or if the specified collection is null
       * @see #remove(Object)
       * @see #contains(Object)
       */
      boolean removeAll(Collection<?> c);
  
      /**
       * Removes all of the elements from this set (optional operation).
       * The set will be empty after this call returns.
       *
       * @throws UnsupportedOperationException if the {@code clear} method
       *         is not supported by this set
       */
      void clear();
  
  
      // Comparison and hashing
  
      /**
       * Compares the specified object with this set for equality.  Returns
       * {@code true} if the specified object is also a set, the two sets
       * have the same size, and every member of the specified set is
       * contained in this set (or equivalently, every member of this set is
       * contained in the specified set).  This definition ensures that the
       * equals method works properly across different implementations of the
       * set interface.
       *
       * @param o object to be compared for equality with this set
       * @return {@code true} if the specified object is equal to this set
       */
      boolean equals(Object o);
  
      /**
       * Returns the hash code value for this set.  The hash code of a set is
       * defined to be the sum of the hash codes of the elements in the set,
       * where the hash code of a {@code null} element is defined to be zero.
       * This ensures that {@code s1.equals(s2)} implies that
       * {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
       * and {@code s2}, as required by the general contract of
       * {@link Object#hashCode}.
       *
       * @return the hash code value for this set
       * @see Object#equals(Object)
       * @see Set#equals(Object)
       */
      int hashCode();
  
      /**
       * Creates a {@code Spliterator} over the elements in this set.
       *
       * <p>The {@code Spliterator} reports {@link Spliterator#DISTINCT}.
       * Implementations should document the reporting of additional
       * characteristic values.
       *
       * @implSpec
       * The default implementation creates a
       * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
       * from the set's {@code Iterator}.  The spliterator inherits the
       * <em>fail-fast</em> properties of the set's iterator.
       * <p>
       * The created {@code Spliterator} additionally reports
       * {@link Spliterator#SIZED}.
       *
       * @implNote
       * The created {@code Spliterator} additionally reports
       * {@link Spliterator#SUBSIZED}.
       *
       * @return a {@code Spliterator} over the elements in this set
       * @since 1.8
       */
      @Override
      default Spliterator<E> spliterator() {
          return Spliterators.spliterator(this, Spliterator.DISTINCT);
      }
  
      /**
       * Returns an immutable set containing zero elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @return an empty {@code Set}
       *
       * @since 9
       */
      static <E> Set<E> of() {
          return ImmutableCollections.Set0.instance();
      }
  
      /**
       * Returns an immutable set containing one element.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the single element
       * @return a {@code Set} containing the specified element
       * @throws NullPointerException if the element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1) {
          return new ImmutableCollections.Set1<>(e1);
      }
  
      /**
       * Returns an immutable set containing two elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if the elements are duplicates
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2) {
          return new ImmutableCollections.Set2<>(e1, e2);
      }
  
      /**
       * Returns an immutable set containing three elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @param e3 the third element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2, E e3) {
          return new ImmutableCollections.SetN<>(e1, e2, e3);
      }
  
      /**
       * Returns an immutable set containing four elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @param e3 the third element
       * @param e4 the fourth element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2, E e3, E e4) {
          return new ImmutableCollections.SetN<>(e1, e2, e3, e4);
      }
  
      /**
       * Returns an immutable set containing five elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @param e3 the third element
       * @param e4 the fourth element
       * @param e5 the fifth element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5) {
          return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5);
      }
  
      /**
       * Returns an immutable set containing six elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @param e3 the third element
       * @param e4 the fourth element
       * @param e5 the fifth element
       * @param e6 the sixth element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6) {
          return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                                 e6);
      }
  
      /**
       * Returns an immutable set containing seven elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @param e3 the third element
       * @param e4 the fourth element
       * @param e5 the fifth element
       * @param e6 the sixth element
       * @param e7 the seventh element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7) {
          return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                                 e6, e7);
      }
  
      /**
       * Returns an immutable set containing eight elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @param e3 the third element
       * @param e4 the fourth element
       * @param e5 the fifth element
       * @param e6 the sixth element
       * @param e7 the seventh element
       * @param e8 the eighth element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8) {
          return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                                 e6, e7, e8);
      }
  
      /**
       * Returns an immutable set containing nine elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @param e3 the third element
       * @param e4 the fourth element
       * @param e5 the fifth element
       * @param e6 the sixth element
       * @param e7 the seventh element
       * @param e8 the eighth element
       * @param e9 the ninth element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9) {
          return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                                 e6, e7, e8, e9);
      }
  
      /**
       * Returns an immutable set containing ten elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @param <E> the {@code Set}'s element type
       * @param e1 the first element
       * @param e2 the second element
       * @param e3 the third element
       * @param e4 the fourth element
       * @param e5 the fifth element
       * @param e6 the sixth element
       * @param e7 the seventh element
       * @param e8 the eighth element
       * @param e9 the ninth element
       * @param e10 the tenth element
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null}
       *
       * @since 9
       */
      static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10) {
          return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                                 e6, e7, e8, e9, e10);
      }
  
      /**
       * Returns an immutable set containing an arbitrary number of elements.
       * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
       *
       * @apiNote
       * This method also accepts a single array as an argument. The element type of
       * the resulting set will be the component type of the array, and the size of
       * the set will be equal to the length of the array. To create a set with
       * a single element that is an array, do the following:
       *
       * <pre>{@code
       *     String[] array = ... ;
       *     Set<String[]> list = Set.<String[]>of(array);
       * }</pre>
       *
       * This will cause the {@link Set#of(Object) Set.of(E)} method
       * to be invoked instead.
       *
       * @param <E> the {@code Set}'s element type
       * @param elements the elements to be contained in the set
       * @return a {@code Set} containing the specified elements
       * @throws IllegalArgumentException if there are any duplicate elements
       * @throws NullPointerException if an element is {@code null} or if the array is {@code null}
       *
       * @since 9
       */
      @SafeVarargs
      @SuppressWarnings("varargs")
      static <E> Set<E> of(E... elements) {
          switch (elements.length) { // implicit null check of elements
              case 0:
                  return ImmutableCollections.Set0.instance();
              case 1:
                  return new ImmutableCollections.Set1<>(elements[0]);
              case 2:
                  return new ImmutableCollections.Set2<>(elements[0], elements[1]);
              default:
                  return new ImmutableCollections.SetN<>(elements);
          }
      }
  }
  