/**
  * Generified Liskov Set example
  * @author Paul Ammann
  */

import java.util.*;

public class LiskovGenericSet<E> {

   private List<E> elements;

   public LiskovGenericSet() {   
      this.elements = new ArrayList<E>();
   }

   public void insert (E e) {
     if (!(isIn(e))) {
        elements.add(e);
     }
   }

   public void remove (E e) {
      elements.remove(e);
   }

   public boolean isIn(E e) {
      return elements.contains(e);
   }

   public int size() {
      return elements.size();
   }

   public E choose() {
      if (elements.size() == 0) throw new IllegalArgumentException();
      return elements.get(0);
   }

   // Conveniently, the rep state is exactly the abstract state
   public String toString() { return elements.toString(); }
}