public void GetItemCount()
{
IMultiSet <string> multiSet = new MultiSet <string>();
multiSet.Add(null, 3);
multiSet.Add("ab");
Assert.AreEqual(3, multiSet.GetItemCount(null));
Assert.AreEqual(1, multiSet.GetItemCount("ab"));
Assert.AreEqual(0, multiSet.GetItemCount("abc"));
}