docs: def collapse(self):
          """
          Collapse the collection items into a single element (list)
  
          :return: A new Collection instance with collapsed items
          :rtype: Collection
          """
          results = []
  
          items = self.items
  
          for values in items:
              if isinstance(values, BaseCollection):
                  values = values.all()
  
              results += values
  
          return self.__class__(results)