svoid testMoreEfficientCopyOnWriteList() {
  new MoreEfficientCopyOnWriteList<Int> l;
  var it = l.iterator();
  l.add(5);
  assertEquals(ll(), asList(it));
  it = l.iterator();
  l.add(0, 10);
  assertEquals(ll(5), asList(it));
  assertEquals(ll(10, 5), asList(l.iterator()));
  
  // make sure this doesn't take O(n^2) time
  time "My list" { repeat 20000 { l.add(0); } }
  
  // ...while this does
  new CopyOnWriteArrayList<Int> l2;
  time "Their list" { repeat 20000 { l2.add(0); } }
}