docs: def append(self, item):
          """Appends a `Monomer to the `Polymer`.
  
          Notes
          -----
          Does not update labelling.
          """
          if isinstance(item, Monomer):
              self._monomers.append(item)
          else:
              raise TypeError(
                  'Only Monomer objects can be appended to an Polymer.')
          return