text: class Stack(object):
      def __init__(self):
          self.__values = []
  
      def push(self, v):
          if v == None:
              raise ValueError("Unsupported value: {0}".format(v))
          self.__values.append(v)
  
      def pop(self):
          if len(self.__values) == 0:
              return None
          else:
              return self.__values.pop()
  
      def len(self):
          return len(self.__values) + 1
  