public void TwoWayDictionary_Add_WhenKeyValueAdded_AddingValueKeyShouldFail()
{
int i = 1;
string s = "one";
_sut.Add(i, s);
_sut.Invoking(x => x.Add(i, "anything else"))
.Should().Throw <ArgumentException>()
.WithMessage($"The collection already contains an item for {i}.");
_sut.Invoking(x => x.Add(666, s))
.Should().Throw <ArgumentException>()
.WithMessage($"The collection already contains an item for {s}.");
}