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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 set(self, key, val):
 """ Return a new PMap with key and val inserted. pmap({'a': 1, 'b': 2}) pmap({'a': 3, 'b': 2}) pmap({'a': 1, 'c': 4, 'b': 2}) """

        return self.evolver().set(key, val).persistent()