public AST_LABEL Pop()
        {
            if (_stack.Count > 0)
            {
                int       index = _stack.Count - 1;
                AST_LABEL lb    = _stack[index];

                _stack.RemoveAt(index);

                return(lb);
            }
            else
            {
                return(null);
            }
        }
 public void Push(AST_LABEL lb) => _stack.Add(lb);