The data formats are described
   three ways: using an ad-hoc UNIX manual page style syntax, using
   augmented BNF described in sections two and three of <A HREF="rfc822.html">RFC 822</A>, and by
   examples. Should there be any conflict between these descriptions,
   the augmented BNF takes precedence. The software behaviors are
   described in words, and specific behavior compliance requirements are
   itemized using the requirements terminology (specifically, the words
   MUST, SHOULD, and MAY) defined 