message: Highlight guard lines in dtx files
  A guard line is a a line consisting of
  %<X...>
  where X can be /,*,-,+,@,< or nothing and ... can hold any character
  valid in an option name.
  Not sure whether to highlight the '%'. For the moment, it is not.
diff: </dict>
  </array>
  </dict>
  + <dict>
  + <key>captures</key>
  + <dict>
  + <key>1</key>
  + <dict>
  + <key>name</key>
  + <string>support.class.guard.dtx</string>
  + </dict>
  + </dict>
  + <key>match</key>
  + <string>^%(\<[/\*\+\<@-]?[A-Za-z0-9]*\>)</string>
  + </dict>
  <dict>
  <key>match</key>
  <string>%</string>
  