// still efficient when used to build a list recursively
persistable sclass ConcatOnDemandList<A> extends RandomAccessAbstractList<A> {
  new LL<A> parts;
  L<A> completeList;
  
  *(L<A>... parts) { fOr (part : parts) addPart(part); }
  
  void addPart(L<A> part) { parts.add(part); }
  
  public int size() { ret completeList().size(); }
  public A get(int i) { ret completeList().get(i); }
  
  L<A> completeList() {
    if (parts != null) {
      new L<A> out;
      for (part : parts)
        collect(part, out);
      parts = null;
      completeList = out;
    }
    ret completeList;
  }
  
  void collect(L<A> part, L<A> out) {
    if (part == null) ret;
    if (part cast ConcatOnDemandList)
      part.collectInto(out);
    else
      out.addAll(part);
  }
  
  void collectInto(L<A> out) {
    if (parts != null)
      for (part : parts)
        collect(part, out);
    else
      out.addAll(completeList);
  }
}