public virtual void Keys()
        {
            #region data

            var idx = new BinaryTreeIndex();

            var n = 10;
            var vertices = CreateKeyValuePairs(n);

            #endregion

            #region test

            idx.AddRange(vertices);

            var keys = idx.Keys();

            Assert.That(keys.LongCount(), Is.EqualTo(n));

            for (long i = 0; i < n; i++)
            {
                CollectionAssert.Contains(keys, i);
            }

            #endregion
        }