/**  an DeptDirectory is either:
 *   (i)  the empty directory new Empty(), or
 *   (ii) the non-empty directory new Cons(Entry,DeptDirectory)
 */
abstract class DeptDirectory {}

class Empty extends DeptDirectory {}

class Cons extends DeptDirectory {
  Entry first;
  DeptDirectory rest;

  /* constructor */
  Cons(Entry f, DeptDirectory r) {
    this.first = f;
    this.rest = r;
  }

  /* accessors */
  Entry getFirst() { return this.first; }
  DeptDirectory getRest() { return this.rest; }	
}