/// <summary>
/// Removes all items from the <see cref="T:System.Collections.Generic.ICollection`1"></see>.
/// </summary>
/// <exception cref="T:System.NotSupportedException">The
/// <see cref="T:System.Collections.Generic.ICollection`1"></see>
/// is read-only.</exception>
public void Clear()
{
_tree = null;
Count = 0;
}