code: def pop_stack(stack, op_id):
    
    if __debug__:
      pushed_stack, pushed_op_id = stack.pop()
      assert pushed_op_id == op_id, 'Wanted %s, got %s' % (op_id, pushed_op_id)
    else:
      pushed_stack = stack.pop()
    return pushed_stack
docs: Proxy of pop, where we know we're popping a stack off of a stack.
  
    We know that we don't need to differentiate through this.
    See pop() for more.
  
    Args:
      stack: The stack to pop from.
      op_id: A unique variable that is also passed into the matching push.
          Allows optimization passes to track pairs of pushes and pops.
  
    Returns:
      The last value.