code: def extend(self, polymer):
          
          if isinstance(polymer, Polymer):
              self._monomers.extend(polymer)
          else:
              raise TypeError(
                  'Only Polymer objects may be merged with a Polymer using unary operator "+".')
          return
docs: Extends the `Polymer` with the contents of another `Polymer`.
  
          Notes
          -----
          Does not update labelling.