old_contents: class Stack(object):
  
      def __init__(self):
          self._items = []
  
      def is_empty(self):
          return not bool(self._items)
  
      def push(self, item):
          self._items.append(item)
  
      def pop(self):
          return self._items.pop()
  
      def peek(self):
          return self._items[-1]
  
      def size(self):
          return len(self._items)
  
new_contents: class Stack:
  
      def __init__(self):
          self._items = []
  
      def is_empty(self):
          return not bool(self._items)
  
      def push(self, item):
          self._items.append(item)
  
      def pop(self):
          return self._items.pop()
  
      def peek(self):
          return self._items[-1]
  
      def size(self):
          return len(self._items)
  
subject: Make the two Stack implementations consistent