old_contents: # TODO: make element as an abstract class
  class Element:
  
      # TODO: use **kwargs
      def __init__(self, name, length):
          self.element_type = name
          self.length = length
  
new_contents: # TODO: make element as an abstract class
  class Element:
      # TODO: use **kwargs
      def __init__(self, name, length):
          self.element_type = name
          self.length = length
          self.families = []
  
  
      def add_to_family(self, family):
          self.families.append(family)
  
subject: Create add_to_family method