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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sequence(self):
 """Returns the sequence of the `Polymer` as a string. Returns ------- sequence : str String of the `Residue` sequence of the `Polypeptide`. """

        seq = [x.mol_letter for x in self._monomers]
        return ''.join(seq)