code: def append(self, item):
          
          if isinstance(item, Polymer):
              self._molecules.append(item)
          else:
              raise TypeError(
                  'Only Polymer objects can be appended to an Assembly.')
          return
docs: Adds a `Polymer` to the `Assembly`.
  
          Raises
          ------
          TypeError
              Raised if other is any type other than `Polymer`.