svoid test_MultiSetMapWithTopTen() {
  new MultiSetMapWithTopTen mm;
  mm.put("a", "b");
  mm.put("a", "c");
  mm.put("b", "e");
  mm.put("b", "f");
  mm.put("b", "g");
  mm.put("c", "x");
  assertEqualsVerbose(ll("b", "a", "c"), asList(mm.keysByPopularityIterator()));
  mm.remove("b", "f");
  assertEqualsVerbose(ll("a", "b", "c"), asList(mm.keysByPopularityIterator()));
  mm.remove("b", "g");
  assertEqualsVerbose(ll("a", "c", "b"), asList(mm.keysByPopularityIterator()));
}