docs: def sf(self,lrt):
          """
          computes the survival function of a mixture of a chi-squared random variable of degree
          0 and a scaled chi-squared random variable of degree d
          """
          _lrt = SP.copy(lrt)
          _lrt[lrt<self.tol] = 0
          pv = self.mixture*STATS.chi2.sf(_lrt/self.scale,self.dof)
          return pv