old_contents: import pytest
  from stack import Element
  from stack import Stack
  
  
  def test_element_init():
      n = Element()
      assert n.val is None
      assert n.next is None
      n = Element(3)
      assert n.val == 3
      assert n.next is None
  
  
  def test_stack_init():
      l = Stack()
      assert l.top is None
  
  
  def test_stack_push():
      l = Stack()
      e = Element(10)
      l.push(e)
      assert l.top == e
      assert l.top.previous is None
      f = Element("String")
      l.push(f)
      assert l.top == f
      assert l.top.previous == e
      assert l.top.previous.previous is None
  
  
  # def test_stack_pop():
  #     l = stack()
  #     a = l.pop()
  #     assert a is None
  
new_contents: import pytest
  from stack import Element
  from stack import Stack
  
  
  def test_element_init():
      n = Element()
      assert n.val is None
      assert n.previous is None
      m = Element(3)
      assert m.val == 3
      assert m.previous is None
  
  
  def test_stack_init():
      l = Stack()
      assert l.top is None
  
  
  def test_stack_push():
      l = Stack()
      e = Element(10)
      l.push(e)
      assert l.top == e
      assert l.top.previous is None
      f = Element("String")
      l.push(f)
      assert l.top == f
      assert l.top.previous == e
      assert l.top.previous.previous is None
  
  
  def test_stack_pop():
      l = Stack()
      e = Element(10)
      l.push(e)
      f = Element("String")
      l.push(f)
      assert l.pop() == f.val
      assert l.top == e
      assert l.top.previous is None
      assert l.pop() == e.val
      assert l.top is None
      with pytest.raises(ValueError):
          l.pop()
  
subject: Add test for pop and adjust element init test