def ncx(wobj, indentarray = [])
      s = @epub.ncx(indentarray)
      if !s.nil? && !wobj.nil?
        wobj.puts s
      end
    end
def ncx(wobj, indentarray = [])
      s = @epub.ncx(indentarray)
      if !s.nil? && !wobj.nil?
        wobj.puts s
      end
    end
Write ncx file to IO object +wobj+. +indentarray+ defines prefix
 string for each level.