class Mail::PartsList

def inspect_structure(parent_id = '')
  enum_for(:map).with_index { |part, i|
    i = i + 1 # Use 1-based indexes since this is for humans to read
    id = parent_id.empty? ? "#{i}" : "#{parent_id}.#{i}"
    if part.content_type == "message/rfc822"
      sub_list = Mail.new(part.body).parts
    else
      sub_list = part.parts
    end
    id + '. ' + part.inspect +
      if sub_list.any?
        "\n" + sub_list.inspect_structure(id)
      end.to_s
  }.join("\n")
end