code: def PredictivePmf(self, xs, name=''):
          
          alpha0 = self.params.sum()
          ps = self.params / alpha0
          return MakePmfFromItems(zip(xs, ps), name=name)
docs: Makes a predictive distribution.
  
          xs: values to go into the Pmf
  
          Returns: Pmf that maps from x to the mean prevalence of x