public interface Sequential 
{ public abstract void append(Object object);
  // adds the given object to end of this sequence
  public int count(Object object);
  // returns the number of occurrences of the 
  // given object that are in this sequence
  
  public abstract Object get(int index);
  // returns the object at the given index,
  // or null if it is not in this sequence
  
  public int getLength();
  // returns the number of elements in this sequence
  
  public int indexOf(Object object);
  // returns the number of elements that come before the
  // first occurrence of the given object in this sequence,
  // or -1 if the given object is not in this sequence
  
  public abstract Object remove(int index);
  // removes and returns the object at the given index,
  // or returns null if index >= length;
  
  public abstract boolean remove(Object object);
  // removes the first occurrence of the the object;
  // returns thre iff successful;
  
  public abstract Object set(int index, Object object);
  // returns the object at the given index after replacing
  // it with the given object, or null is unsuccessful 

  public String toString();
  // returns a string that displays the contents of this
  // sequence
}