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# File log4r/outputter/outputter.rb, line 44
    def only_at(*levels)
      raise ArgumentError, "Gimme some levels!", caller if levels.empty?
      raise ArgumentError, "Can't log only_at ALL", caller if levels.include? ALL
      levels.each {|level| Log4rTools.validate_level(level)}
      @level = levels.sort.first
      OutputterFactory.create_methods self, levels
      Logger.log_internal {
        "Outputter '#{@name}' writes only on " +\
        levels.collect{|l| LNAMES[l]}.join(", ")
      }
    end