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  def header(context, name, value)
    @values ||= {}
    case name
    when /\A(Subject|From|To)\z/i
      key = $1.to_s.downcase
      utf8_value = NKF.nkf("-w", value)
      @values[key] = utf8_value
    when /\AContent-Transfer-Encoding\z/i
      @encoding = value
    end
  end