<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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