svoid test_TreeSetWithDuplicates() {
  TreeSetWithDuplicates<S> set = new(comparatorFromCalculatedField l());
  set.addAll(ll("hello", "world", "bla", "very long"));
  assertEqualsOneOfVerbose(asList(set),
    ll("bla", "hello", "world", "very long"),
    ll("bla", "world", "hello", "very long"));
  assertEqualsVerbose(4, set.size());
  
  // test all cases of size update
  set.remove("x"); // remove element not in set
  assertEqualsVerbose(4, set.size());
  set.remove("bla"); // remove element without duplicates
  assertEqualsVerbose(3, set.size());
  set.remove("hello"); // remove element with a remaining duplicate
  assertEqualsVerbose(2, set.size());
  set.add("world"); // add existing element
  assertEqualsVerbose(2, set.size());
  set.add("old"); // add new element
  assertEqualsVerbose(3, set.size());
  set.add("new"); // add new element with existing duplicate
  assertEqualsVerbose(4, set.size());
  
  test_TreeSetWithDuplicates_2();
}

// test that elements with same probability are returned in
// insertion order
svoid test_TreeSetWithDuplicates_2() {
  TreeSetWithDuplicates<S> set = new(comparatorFromCalculatedField l());
  LS l = ll("abc", "xyz", "def", "ppp", "aaa");
  set.addAll(l);
  assertEqualsVerbose(l, asList(set));
}