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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 synsets(self, lemma, pos = None):
        '''
        Looks up synsets in the GermaNet database.

        Arguments:
        - `lemma`:
        - `pos`:
        '''
        return sorted(set(lemma_obj.synset
                          for lemma_obj in self.lemmas(lemma, pos)))