/// <summary>Gets the key at the supplied index.</summary>
/// <param name="index">The zero-based index of the key to get.</param>
/// <returns>The key at <em>index</em>.</returns>
/// <remarks>This is a O(log <em>n</em>) operation.</remarks>
/// <exception cref="ArgumentOutOfRangeException">When <em>index</em> is less than zero or greater than or equal to the number of keys.</exception>
public TKey ElementAt(int index)
{
if (index < 0 || index >= Count)
{
throw new ArgumentOutOfRangeException(nameof(index), "Argument is out of the range of valid values.");
}
var leaf = tree.Find(index, out int leafIndex);
return(leaf.GetKey(leafIndex));
}