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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 application_properties(self, key=None):
 """Return the mutable server application properties. :param key: the single property to return a value for :type key: Optional[str] :rtype: Union[Dict[str, str], List[Dict[str, str]]] """

        params = {}
        if key is not None:
            params['key'] = key
        return self._get_json('application-properties', params=params)