code: def derivative(self, vs=None):
          r
          return functools.reduce(operator.xor, self.iter_cofactors(vs))
docs: 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*.