////   This file contains the C# code from Program 4.9 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/pgm04_09.txt//public abstract class Matrix{    public abstract double this[int i, int j] { get; set; }    public abstract int Rows { get; }    public abstract int Columns { get; }    public abstract Matrix Transpose { get; }    public abstract Matrix Plus(Matrix matrix);    public abstract Matrix Times(Matrix matrix);    public static Matrix operator +(Matrix m1, Matrix m2)	{ return m1.Plus(m2); }    public static Matrix operator *(Matrix m1, Matrix m2)	{ return m1.Times(m2); }    // ...}