sclass TokenIndexedList extends RandomAccessAbstractList<S> {
  new Map<S, Token> index; // first occurrence of token
  new L<Token> list;
  
  sclass Token {
    S s;
    Token prevSame, nextSame;
  }
  
  public S get(int i) { ret list.get(i).s; }
  public int size() { ret list.size(); }
  public void set(int i, S s) {
    Token t = list.get(i);
    if (eq(t.s, s)) ret;
    
    // remove from chain
    if (t.prevSame == null)
      index.put(t.s, t.nextSame);
    else
      t.prevSame.nextSame = t.nextSame;
    if (t.nextSame != null)
      t.nextSame.prevSame = t.nextSame;
      
    // change token, add to chain
    Token t2 = index.get(s);
    t.s = s;
    ...
  }
}