anchor.diff:      each lesson in lessons
         if lesson.indexed !== false
           //- if external tag set, use that as link
  -        -link = lesson.external || lesson.link
  -        a(href=relative(link))
  +        -link = lesson.external || relative(lesson.link)
  +        a(href=link)
             li.lesson= lesson.title
   
   
  
positive.code: 
    each lesson in playlist.lessons
      //- if external tag set, use that as link
      -link = lesson.external || lesson.link
      a(href=relative(link))
        li.playlist.lesson(style="display:none" class=playlist.id + ' level-' + lesson.level)
          = lesson.title
  
  
negative.code:   -levels = _.uniq(_.compact(_.pluck(collections[path.dir], 'level'))).sort()
      each level in levels
        +levelIndex(level, _.where(collections[path.dir], {level: level}))
  