code: def pdf(self, mu):
          
          if self.transform is not None:
              mu = self.transform(mu)                
          return ss.expon.pdf(mu, self.lmd0)
docs: PDF for Exponential prior
  
          Parameters
          ----------
          mu : float
              Latent variable for which the prior is being formed over
  
          Returns
          ----------
          - p(mu)