def add_variable(self, v):
self.variables[v]<EOL>
Add a variable.

        Args:
            v (variable):
                Variable in the constraint satisfaction problem. May be of any type that
                can be a dict key.

        Examples:
            This example adds two variables, one of which is already used in a constraint
            of the constraint satisfaction problem.

            >>> import dwavebinarycsp
            >>> import operator
            >>> csp = dwavebinarycsp.ConstraintSatisfactionProblem(dwavebinarycsp.SPIN)
            >>> csp.add_constraint(operator.eq, ['a', 'b'])
            >>> csp.add_variable('a')  # does nothing, already added as part of the constraint
            >>> csp.add_variable('c')
            >>> csp.check({'a': -1, 'b': -1, 'c': 1})
            True
            >>> csp.check({'a': -1, 'b': -1, 'c': -1})
            True