docs: def trim(self, restriction_list):
          """
          Trim and reorder the attributes to the specifications in a restriction
          list.
          """
          replacement = []
          for reference in restriction_list:
              replacement.append(self.get_attribute(reference))
  
          if self.has_duplicates(replacement):
              raise AttributeReferenceError('Duplicate attribute reference.')
          self._contents = replacement