@abc.abstractmethod<EOL><INDENT>def is_value_known(self):<DEDENT>
return<EOL>
Tests if an existing value is known for this data element.

        If it is known that a value does not exist, then this method
        returns ``true``.

        :return: ``true`` if the element value is known, ``false`` if the element value is not known
        :rtype: ``boolean``


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