@abc.abstractmethod<EOL><INDENT>def get_num_days(self):<DEDENT>
return<EOL>
Gets the number of days in a year.

        For a variable day calendar, the number of all defined days are
        returned. If there are no "days" in this calendar system then
        this value may be zero. If there are no "months" defined then
        the number of days is the number of days in a year.

        :return: the number of days
        :rtype: ``cardinal``


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