Extends the `Polymer` with the contents of another `Polymer`.

        Notes
        -----
        Does not update labelling.

def extend(self, polymer):
        """Extends the `Polymer` with the contents of another `Polymer`.

        Notes
        -----
        Does not update labelling.
        """
        if isinstance(polymer, Polymer):
            self._monomers.extend(polymer)
        else:
            raise TypeError(
                'Only Polymer objects may be merged with a Polymer using unary operator "+".')
        return