[BCP137]   Best Current Practice 137,
              <https://www.rfc-editor.org/info/bcp137>.
              At the time of writing, this BCP comprises the following:

              Klensin, J., "ASCII Escaping of Unicode Characters",
              BCP 137, RFC 5137, DOI 10.17487/RFC5137, February 2008,
              <https://www.rfc-editor.org/info/rfc5137>.