/// <summary>
/// Updates the element at <paramref name="index"/> using <paramref name="function"/>.
/// </summary>
/// <param name="index">The index.</param>
/// <param name="function">The function to apply to the element currently at <paramref name="index"/></param>
/// <exception cref="ArgumentOutOfRangeException"><c>index</c> is out of range.</exception>
/// <remarks>
/// Equivalent to <code>SetAt(index, function(this[index])), but faster.</code>
/// </remarks>
public RandomAccessSequence <T> AdjustAt(int index, Func <T, T> function)
{
Requires.That.IsIndexInRange(this, index, "index").Check();
var split = _ft.SplitTreeAt(index, true, true);
T currentValue = split.Pivot.Value;
return(new RandomAccessSequence <T>(
(split.Left | new Element(function(currentValue))) + split.Right));
}
