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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def derivative(self, vs=None):
 r"""Return the derivative of a function over a sequence of N variables. The *vs* argument is a sequence of :math:`N` Boolean variables. The *derivative* of :math:`f(x_1, x_2, \dots, x_i, \dots, x_n)` with respect to variable :math:`x_i` is: :math:`\frac{\partial}{\partial x_i} f = f_{x_i} \oplus f_{x_i'}` This is also known as the Boolean *difference*. """

        return functools.reduce(operator.xor, self.iter_cofactors(vs))