# File lib/graphviz/math/matrix.rb, line 50
      def [](line, column)
        unless (0...@line).to_a.include?(line-1)
          raise CoordinateError, "Line out of range (#{line} for 1..#{@line})!"
        end
        unless (0...@column).to_a.include?(column-1)
          raise CoordinateError, "Column out of range (#{column} for 1..#{@column})!"
        end
        @matrix[line-1][column-1]
      end