def to_xml_string(str='')
      return if empty?
      str << ('<rowBreaks count="' << self.size.to_s << '" manualBreakCount="' << self.size.to_s << '">')
      each { |brk| brk.to_xml_string(str) }
      str << '</rowBreaks>'
    end
def to_xml_string(str='')
      return if empty?
      str << ('<rowBreaks count="' << self.size.to_s << '" manualBreakCount="' << self.size.to_s << '">')
      each { |brk| brk.to_xml_string(str) }
      str << '</rowBreaks>'
    end
<rowBreaks count="3" manualBreakCount="3">
 <brk id="1" max="16383" man="1"/>
 <brk id="7" max="16383" man="1"/>
 <brk id="13" max="16383" man="1"/>
 </rowBreaks>