def comments(sheet = nil)
      sheet ||= default_sheet
      read_comments(sheet) unless @comments_read[sheet]
      return [] unless @comment[sheet]
      @comment[sheet].each.collect do |elem|
        [elem[0][0], elem[0][1], elem[1]]
      end
    end
def comments(sheet = nil)
      sheet ||= default_sheet
      read_comments(sheet) unless @comments_read[sheet]
      return [] unless @comment[sheet]
      @comment[sheet].each.collect do |elem|
        [elem[0][0], elem[0][1], elem[1]]
      end
    end
returns each comment in the selected sheet as an array of elements
 [row, col, comment]