new_contents: class Level(object):
      def __init__(self, data, under=None):
          self.data = data
          self.under = under
  
  
  class Stack(object):
      def __init__(self):
          self.top = None
          self._size = 0
  
      def push(self, val):
          self.top = Level(val, self.top)
          self._size += 1
  
      def pop(self):
          if self._size == 0:
              #raise error
              pass
          result = self.top
          self.top = self.top.under
          return result
  
subject: create stack class with level class with pop and push functions