public interface Document {
    
    // Pre: None
    // Post: Returns the number of characters in this Document
    public int size();
    
    // Pre: size() > 0
    // Post: Returns a Cursor pointing to the first character in this Document
    public Cursor start();
    
    // Pre: size() > 0
    // Post: Returns a Cursor pointing to the last character in this Document
    public Cursor end();

    // Pre: None
    // Post: last character in this Document is c; size() = old size() + 1
    public void append(char c);

    // Pre: None
    // Post: Builds a new String with the same characters as are found in this
    //       Document
    public String toString();
}