code: def smoothing(self, vs=None):
          r
          return functools.reduce(operator.or_, self.iter_cofactors(vs))
docs: r"""Return the smoothing of a function over a sequence of N variables.
  
          The *vs* argument is a sequence of :math:`N` Boolean variables.
  
          The *smoothing* of :math:`f(x_1, x_2, \dots, x_i, \dots, x_n)` with
          respect to variable :math:`x_i` is:
          :math:`S_{x_i}(f) = f_{x_i} + f_{x_i'}`
  
          This is the same as the existential quantification operator:
          :math:`\exists \{x_1, x_2, \dots\} \: f`