////   This file contains the C# code from Program 10.21 of//   "Data Structures and Algorithms//    with Object-Oriented Design Patterns in C#"//   by Bruno R. Preiss.////   Copyright (c) 2001--2002 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.brpreiss.com/books/opus6/programs/pgm10_21.txt//public class BTree : MWayTree{    protected BTree parent;    public override void Insert(ComparableObject obj)    {	if (IsEmpty)	{	    if (parent == null)	    {		AttachSubtree(0, new BTree(M));		key[1] = obj;		AttachSubtree(1, new BTree(M));		count = 1;	    }	    else		parent.InsertPair(obj, new BTree(M));	}	else	{	    int index = FindIndex(obj);	    if (index != 0 && obj == key[index])		throw new ArgumentException("duplicate key");	    subtree[index].Insert(obj);	}    }    // ...}