code: def MarginalBeta(self, i):
          
          alpha0 = self.params.sum()
          alpha = self.params[i]
          return Beta(alpha, alpha0 - alpha)
docs: Computes the marginal distribution of the ith element.
  
          See http://en.wikipedia.org/wiki/Dirichlet_distribution
          #Marginal_distributions
  
          i: int
  
          Returns: Beta object