text: class Slide:
  
      """
      Combination of images represented as a slide
  
      List of attributes:
  
      - photos:list = list of photos the second being None if is a horizontal pic
      - is_vertical:bool = True if the slide is composed by two vertical images
      - tags:set = set being the union of both set of images
  
  
      """
  
      def __init__(self, photo1, photo2 = None):
  
          """
          photo1: Photo
          photo2: Photo (only if we have two vertical photos)
          """
  
          # Check that the photo is vertical
          if photo2:
              assert(photo2.is_vertical)
  
          # List of the photos
          self.photos = [photo1, photo2]
  
          
          self.is_vertical = all(self.photos)
      
      @property
      def tags(self):
          
          result = set()
  
          for photo in self.photos:
              if photo:
                  result.union(photo.tags)
  
          return result
  
      def pk(self):
          pk1 = photo1.pk
          pk2 = 0
          if photo2:
              pk2 = photo2.pk*1e6
  
          return pk1+pk2
  