def comment(row, col, sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      row, col = normalize(row, col)
      return nil unless @comment[sheet]
      @comment[sheet][[row, col]]
    end
def comment(row, col, sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      row, col = normalize(row, col)
      return nil unless @comment[sheet]
      @comment[sheet][[row, col]]
    end
returns the comment at (row/col)
 nil if there is no comment