def opf(wobj)
      s = @epub.opf
      if !s.nil? && !wobj.nil?
        wobj.puts s
      end
    end
def opf(wobj)
      s = @epub.opf
      if !s.nil? && !wobj.nil?
        wobj.puts s
      end
    end
Write opf file to IO object +wobj+.