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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 get_string(self, key, default=UndefinedKey):
 """Return string representation of value found at key :param key: key to use (dot separated). E.g., a.b.c :type key: basestring :param default: default value if key not found :type default: basestring :return: string value :type return: basestring """

        value = self.get(key, default)
        if value is None:
            return None

        string_value = unicode(value)
        if isinstance(value, bool):
            string_value = string_value.lower()
        return string_value