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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 incr(self, key, delta=1):
 """ Add delta to value in the cache. If the key does not exist, raise a ValueError exception. """

        value = self.get(key)
        if value is None:
            raise ValueError("Key '%s' not found" % key)
        new_value = value + delta
        self.set(key, new_value)
        return new_value