RFC 2119
RFC 2119 RFCにおいて要請の程度を示すために用いるキーワード
https://www.nic.ad.jp/ja/tech/ipa/RFC2119JA.html
RFC 8174
https://tex2e.github.io/rfc-translater/html/rfc8174.html
MUST
/ REQUIRED / SHALL
MUST NOT
/ SHALL NOT
SHOULD
/ RECOMMENDED
SHOULD NOT
/ NOT RECOMMENDED
MAY
/ OPTIONAL