def append(self, clause):
        """
            Add one more clause to CNF formula. This method additionally
            updates the number of variables, i.e. variable ``self.nv``, used in
            the formula.

            :param clause: a new clause to add.
            :type clause: list(int)

            .. code-block:: python

                >>> from pysat.formula import CNF
                >>> cnf = CNF(from_clauses=[[-1, 2], [3]])
                >>> cnf.append([-3, 4])
                >>> print cnf.clauses
                [[-1, 2], [3], [-3, 4]]
        """

        self.nv = max([abs(l) for l in clause] + [self.nv])
        self.clauses.append(clause)