public void Add()
{
var lookup = new MutableLookup<string, string>();
lookup.Add ("F", "Foo");
Assert.AreEqual (1, lookup.Count);
Assert.AreEqual ("Foo", lookup["F"].First());
lookup.Add ("F", "Foobar");
Assert.AreEqual (1, lookup.Count);
Assert.AreEqual (2, lookup["F"].Count());
}