code: def rand_student_t(df, mu=0, std=1):
      
  
      x = random.gauss(0, std)
      y = 2.0*random.gammavariate(0.5 * df, 2.0)
      return x / (math.sqrt(y / df)) + mu
docs: return random number distributed by student's t distribution with
      `df` degrees of freedom with the specified mean and standard deviation.