/**  An interface for objects for which comparison
       by lexicographical order is sensible.  The
       objects are assumed to be sequences of
       elements drawn from a fixed finite sets called
       an <code>alphabet</code>.  Often the same
       alphabet is used for each position in the
       sequence, but this is not required by the interface.
       Alphabets are assumed to be ordered, so that one
       may speak of the 0th, 1st, etc., elements of
       the alphabet.   The 0th element is assumed
       to be a terminator, to be returned for
       example when seeking the kth element of
       a string of length k-1 or k-2.
     However, no ordering methods are assumed either
       for the objects themselves, or for the members of
       their underlying alphabets.
     Lexicographical objects are assumed to know the
       size of the alphabet from which they are
       composed.  The interface provides a method
       to access this value.  If there are different
       alphabets for different positions of the sequence,
       then the maximum of these alphabet sizes is to
       be returned -- there is no provision for finding
       the alphabet size at a given position.
*/


public interface Lexicographical
{

  /**
     @return the number of elements in
        the Lexicographical object
  */
  public int length();


  /**
     @param i a position in a lexicographical object
     @return an integer code representing
       the character at position i.  Assumes that
        positions start at 0.  Returns 0 if
        there is no character at the given position.
  */
  public int elementAt(int i);


  /**
     @return the number of distinct characters
       in the alphabet, including the terminator
       character.
  */
  public int getAlphabetSize();

};