code: def is_neg_unate(self, vs=None):
          r
          vs = self._expect_vars(vs)
          basis = self.support - set(vs)
          maxcov = [PC_ONE] * (1 << len(basis))
          # Test whether table entries are monotonically decreasing
          for cf in self.iter_cofactors(vs):
              for i, item in enumerate(cf.pcdata):
                  if maxcov[i] == PC_ZERO and item == PC_ONE:
                      return False
                  maxcov[i] = item
          return True
docs: r"""Return whether a function is negative unate.
  
          A function :math:`f(x_1, x_2, ..., x_i, ..., x_n)` is *negative unate*
          in variable :math:`x_i` if :math:`f_{x_i'} \geq f_{xi}`.