////   This file contains the C# code from Program 10.20 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_20.txt//public class BTree : MWayTree{    protected BTree parent;    public BTree(int m) : base(m)	{}    public virtual void AttachSubtree(int i, MWayTree arg)    {	BTree btree = (BTree)arg;	subtree[i] = btree;	btree.parent = this;    }    // ...}