@abc.abstractmethod<EOL><INDENT>def get_num_months(self):<DEDENT>
return<EOL>
Gets the number of months of the year.

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

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


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