import java.io.*;import java.lang.*;import java.util.*;public class ItemSet extends HashSet{  ItemSet()  {       super();  }  ItemSet( ItemSet s )  {    super(s);  }  public ItemSet union(ItemSet s)  {    ItemSet set = new ItemSet(this);    set.addAll( s );  return set;  }    public void unionWith(ItemSet s)  {    this.addAll(s);  }  public ItemSet difference(ItemSet s)  {    ItemSet set = new ItemSet(this);    set.removeAll(s);  return set;  }  public void differenceWith(ItemSet s)  {    this.removeAll(s);  }  public ItemSet intersection(ItemSet s)  {    ItemSet set = new ItemSet(this);    set.retainAll(s);  return set;  }  public void intersectionWith(ItemSet s)  {    this.retainAll(s);  }  public boolean subsetOf(ItemSet s)  {    return( s.containsAll(this) );  }      public void copy(ItemSet s)  {    this.union(s);  }}