    # 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