how to mix $or and $and with QueryEngine? (http://bevry.me/queryengine/guide)

test.coffee

# query blog items and main nav items
# this doesn't work, I only get the blog items

findFullNav: (lang) ->
  @getDatabase().findAllLive
    $or:
      $and: 
        mainNav: {$exists: true}
        relativeOutDirPath:
          $startsWith: lang
      $and:
        category: {$exists: true}
        relativeOutDirPath:
          $startsWith: "#{lang}/blog"