Return a newly added CT_Num (<w:num>) element referencing the
        abstract numbering definition identified by *abstractNum_id*.

def add_num(self, abstractNum_id):
        """
        Return a newly added CT_Num (<w:num>) element referencing the
        abstract numbering definition identified by *abstractNum_id*.
        """
        next_num_id = self._next_numId
        num = CT_Num.new(next_num_id, abstractNum_id)
        return self._insert_num(num)