code: def CredibleInterval(self, percentage=90):
          
          prob = (1 - percentage / 100.0) / 2
          interval = self.Value(prob), self.Value(1 - prob)
          return interval
docs: Computes the central credible interval.
  
          If percentage=90, computes the 90% CI.
  
          Args:
              percentage: float between 0 and 100
  
          Returns:
              sequence of two floats, low and high