// should be thread-safe by nature
// compare efficiently due to md5 caching
// tested with A = S
static class ImmL<A> extends AbstractList<A> {
  new L<A> l;
  int hash;
  S md5;
  
  *() {}
  *(L<A> _l) { l.addAll(_l); }
  
  // List methods
  
  public int size() {
    ret l.size();
  }
  
  public A get(int i) {
    ret l.get(i);
  }
  
  public synchronized int hashCode() {
    if (hash == 0)
      hash = l.hashCode();
    ret hash;
  }
  
  public bool equals(O o) {
    if (o instanceof ImmL
      && neq(md5(), ((ImmL) o).md5()))
      false;
    ret super.equals(o);
  }
  
  public synchronized S md5() {
    if (md5 == null)
      md5 = main.md5(structure(l));
    ret md5;
  }
}