public FractionMatrix multiply(FractionMatrix that)
    {
        Fraction sum = new Fraction( 0, 1 );
        if (this.numberColumns() != that.numberRows())
            throw new MatrixException("incompatible matrix dimensions");
        Fraction[][] product = new Fraction[this.numberRows()][that.numberColumns()];
        FractionMatrix tempProduct = new FractionMatrix( product );
        for (int row = 0; row < tempProduct.numberRows(); row++)
        {
            for (int col = 0; col < tempProduct.numberColumns(); col++)
            {
                for( int a = 0; a < this.numberRows(); a++ )
                {
                    for( int b = 0; b < that.numberColumns(); b++ )
                    {
                        sum = sum.add( this.get( a, b ).multiply( that.get( b, a ) ) );
                    }
                }
                product[row][col] = sum;
                sum = new Fraction( 0 , 1 );
            }
        }
        return new FractionMatrix( product );
    }