public interface Set {

   public void insert(Element x);
   public void delete(Element x);
   /**
     * Find and return the Element x in this set if there;  otherwise null.
     * Note that x may match (by elEquals) an element in this set, even
     * though x is not equal to that element.  Thus, this method can return
     * a different element than x---one that matches x through elEquals but
     * does not == x.
     */
   public Element find(Element x);
   /**
     * Does this Set have Element x?
     * Possible implementation:  find(x) != null
     */
   public boolean hasElement(Element x);
   public void union(Set other);
   public void intersection(Set other);
   public void difference(Set other);
   /**
     * Return the elements of this set as an array.
     * The elements need not show up in any particular order.
     */
   public Element[] getContents();
   /**
     * Returns the number of elements in this set.
     * Possible implementation:  getContents().length
     *
     */
   public int sizeOf();
   /**
     * Is this set empty?
     * Possible implementation:  sizeOf() == 0
     */
   public boolean isEmpty();
}