code: def CredibleInterval(pmf, percentage=90):
      
      cdf = pmf.MakeCdf()
      prob = (1 - percentage / 100.0) / 2
      interval = cdf.Value(prob), cdf.Value(1 - prob)
      return interval
docs: Computes a credible interval for a given distribution.
  
      If percentage=90, computes the 90% CI.
  
      Args:
          pmf: Pmf object representing a posterior distribution
          percentage: float between 0 and 100
  
      Returns:
          sequence of two floats, low and high