To go beyond generic Wiki syntax, SGML lets you define context-specific token replacement rules. For example, to make SGML format a simplistic markdown fragment into HTML, you could use an SGML prolog like this:

    <!DOCTYPE p [
      <!ELEMENT p - - ANY>
      <!ELEMENT em - - (#PCDATA)>
      <!ENTITY start-em '<em>'>
      <!ENTITY end-em '</em>'>
      <!SHORTREF in-p '*' start-em>
      <!SHORTREF in-em '*' end-em>
      <!USEMAP in-p p>
      <!USEMAP in-em em>
    ]>
    <p>The following text:
       *this*
       will be put into EM
       element tags</p>