/* ObjectList ::= EmptyObjectList | ConsObjectList where ConsObjectList includes an Object and an ObjectList. */
abstract class ObjectList { 
  /** Adds the element n to the front of this ObjectList. */
  ObjectList cons(Object n) { return new ConsObjectList(n, this); }
   
  /** Returns a String representation of this ObjectList in Scheme format: (e1 ... en) . */
  abstract String listString();
  /** Returns s String containing the elements of this ObjectList, where each element is preceded by ' '.  Hence,
    * (e1 e2... en) generates " e1 e2 ... en" */ 
  abstract String listStringHelp();
}

class EmptyObjectList extends ObjectList {
  /** Field containing the only (singleton) EmptyObjectList. */
  static EmptyObjectList ONLY = new EmptyObjectList();
  /** Private constructor used to construct a singleton. */
  private EmptyObjectList() { }
  
  String listString() { return "()"; }
  String listStringHelp() { return ""; }
}

class ConsObjectList extends ObjectList {
  /** The first element of this ObjectList. */
  Object first;
  /** The remaining elements of this ObjectList. */
  ObjectList rest;
  
  String listString() { return "(" + first + rest.listStringHelp() + ")"; }
  String listStringHelp() { return " " + first + rest.listStringHelp(); }
}