text: class Stack(object):
      def __init__(self):
          self.__values = []
          self.__len = 0
  
      def push(self, v):
          self.__len += 1
          if v == None:
              raise ValueError("Unsupported value: {0}".format(v))
          self.__values.append(v)
          
      def pop(self):
          if not self.__values:
              return None
          else:
              self.__len -= 1
              return self.__values.pop()
  
      def len(self):
          return self.__len
  
  