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