new_contents: import rt
  
  class Factorial(rt.Actor):
  
      def __init__(self):
          super().__init__()
          self.behavior = self.factorial_beh
  
      def factorial_beh(self, message):
          customer, n = message
          if n == 0:
              customer(1)
          else:
              multiply_by_n = Multiplier.create(customer, n)
              self((multiply_by_n, n-1))
  
  
  class Multiplier(rt.Actor):
  
      def __init__(self, customer, n):
          super().__init__()
          self.customer = customer
          self.n = n
          self.behavior = self.multiplier_beh
  
      def multiplier_beh(self, m):
          self.customer(m * self.n)
          
subject: Add factorial example