class State        // Linked list of the current values of variables.
  {
  String name;     // Variable's name.
  int    value;    // Its value.
  State  next;     // Next entry in list.
  
  State() { name = "<end>"; }     // First entry is a dummy.
  State(String n, int v, State s) { name = n; value = v; next = s; }
  
  // Do a lookup of variable named n, returning its value.
  int valueOf(String n)   
    {
    for (State S = this; S != null; S = S.next)
      if (n.compareTo(S.name) == 0) 
        return S.value;
    return 0;  // Probably not a good idea.
    }
     
  // Change variable v to have value n, or add a new entry if necessary.
  State update(String n, int v)    
    {
    for (State S = this; S != null; S = S.next)
      if (n.compareTo(S.name) == 0)
        {
        S.value = v;  // New value for existing variable.
        return this;
        }
    return new State(n, v, this);  // Add an entry to this state.
    }
  
  public String toString()
    {
    String result = "";
    for (State S = this; S != null; S = S.next)
      result += S.name + " = " + S.value + "\n";
    return result;
    }
  }