// Textbook fragment 06.18
/**
 * An interface for a sequence, a data structure supporting all
 * operations of a deque, indexed list and position list.
 */
public interface Sequence<E>
     extends Deque<E>, IndexList<E>, PositionList<E> {
  /** Returns the position containing the element at the given index. */
  public Position<E> atIndex(int r) throws BoundaryViolationException;
  /** Returns the index of the element stored at the given position. */
  public int indexOf(Position<E> p) throws InvalidPositionException;
}