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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 sub(self, key):
 """Returns new Vyper instance representing a sub tree of this instance. """

        subv = Vyper()
        data = self.get(key)
        if isinstance(data, dict):
            subv._config = data
            return subv
        else:
            return None