@abc.abstractmethod<EOL><INDENT>def add_translation(self, source_text, target_text):<DEDENT>
pass<EOL>
Adds or updates a string translation.

        :param source_text: the source ``string``
        :type source_text: ``string``
        :param target_text: the translated string
        :type target_text: ``string``
        :raise: ``NullArgument`` -- ``source_text`` or ``target_text`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method must be implemented.*