public void Keys_ReturnsCollectionOfKeys()
        {
            var sourceKeys = new[] { "A", "B", "C" };

            _target.Add(sourceKeys[0], "B");
            _target.Add(sourceKeys[1], "C");
            _target.Add(sourceKeys[2], "D");

            var keys = _target.Keys();

            Assert.All(keys, key => sourceKeys.Contains(key));
            Assert.All(sourceKeys, key => keys.Contains(key));
        }