# File lib/graphviz/math/matrix.rb, line 96
      def -(m)
        matrix = GraphViz::Math::Matrix.new( @line, @column )
        @line.times do |line|
          @column.times do |column|
            matrix[line+1, column+1] = self[line+1, column+1] - m[line+1, column+1]
          end
        end
        return matrix
      end