static class HybridTokens extends AbstractList<S> {
  Tokenization tok;
  L<LinkedToken> pointers;
  static bool debug;
  static int instances;
  
  sclass LinkedToken {
    S t;
    int index = -1;
    LinkedToken prev, next;
    LinkedToken prevIdentical, nextIdentical;
    
    *() {}
    *(S *t) {}
  }
  
  sclass Tokenization {
    LinkedToken first, last;
    new Map<S, LinkedToken> firstByContent;
    new Map<S, LinkedToken> lastByContent;
    
    bool contains(S t) {
      ret firstByContent.get(t) != null;
    }
    
    int countInstances(S t) {
      int n = 0;
      LinkedToken lt = firstByContent.get(t);
      while (lt != null) {
        ++n;
        lt = lt.nextIdentical;
      }
      ret n;
    }
  }

  *() { ++instances; }
  *(L<S> tokens) {
    this();
    tok = makeTokenization(tokens);
    pointers.ensureCapacity(l(tokens));
  }
  
  // required immutable list methods
  
  @Override
  public S get(int i) {
    int j = max(size(),
  }
  
  @Override
  public int size() {
    if (last(pointers) != tok.last) {
      LinkedToken lt = last(pointers);
      if (lt == null) lt = tok.first;
      if (lt != null)
        while ((lt = lt.next) != null)
          pointers.add(lt);
    }
    ret l(pointers);
  }
  
  // required mutable list methods
  
  @Override
  public S set(int i, S a) {
    LinkedToken lt = get(i);
    if (eq(lt.t, a)) ret;
    
    // Take out of old content chain
    if (lt.prevIdentical == null) tok.firstByContent.remove(a);
    else lt.prevIdentical.nextIdentical = lt.nextIdentical;
    if (lt.nextIdentical == null) tok.lastByContent.remove(a);
    else lt.nextIdentical.prevIdentical = lt.prevIdentical;
    
    // Add to new content chain
    lt.prevIdentical = lt_findPrevIdentical(lt);
    if (lt.prevIdentical == null) tok.firstByContent.put(a, lt);
    lt.nextIdentical = lt_findNextIdentical(lt);
    if (lt.nextIdentical == null) tok.lastByContent.put(a, lt);
    
    // Set new value and return old one
    S prev = lt.t;
    lt.t = a;
    ret prev;
  }
  
  @Override
  public void add(int i, S a) {
    LinkedToken lt = get(i);
    if (lt == null) {
      assertTrue(i == size());
      lt_addToken(tok, a);
    } else {
      LinkedToken ltNew = new(a);
      ltNew.last = lt.last;
      if (lt.last != null) lt.last.next = ltNew; else tokens.first = ltNew;
      ltNew.next = lt;
      lt.last = ltNew;
      lt_addToContentChain(lt, tok);
      clearPointersFrom(i);
    }
  }
  
  @Override
  public S remove(int i) {
    S a = l.get(i);
    index.remove(a);
    l.remove(i);
    ret a;
  }
  
  // speed up methods

  @Override  
  protected void removeRange(int fromIndex, int toIndex) {
    for (int i = fromIndex; i < toIndex; i++)
      unindex(i);
    l.subList(fromIndex, toIndex).clear();
  }
  
  @Override
  public int indexOf(O a) {
    LinkedToken lt = tok.firstByContent(a);
    if (lt == null) ret -1;
    makeIndex(lt);
    ret lt.index;
  }
  
  @Override
  public bool contains(O a) {
    ret tok.contains((S) a);
  }
  
  @Override
  public void clear() {
    index.clear();
    l.clear();
  }
  
  @Override
  public bool addAll(int i, Collection<? extends S> c) {
    index.addAll((Collection) c);
    ret l.addAll(i, c);
  }
  
  // indexing methods
  
  void makeIndex(LinkedToken lt) {
    if (lt.index >= 0) ret;
    LinkedToken l = lt.prev;
    while (l != null && l.index < 0) l = l.prev;
    int index = 0;
    if (l == null) l = tok.first;
    while true {
      l.index = index++;
      if (l == lt) break;
      l = l.next;
    }
  }
  
  // static methods
  
  static HybridTokens ensureIndexed(L<S> l) {
    ret l instanceof HybridTokens ? (HybridTokens) l : new HybridTokens(l);
  }
}

Began life as a copy of #1004045