streams and trees in elisp using loop

*scratch*.el

(let (stack
      result
      (stream '((START "a")
                (START "b")
                (START "c")
                (TEXT "test 1")
                (END "c")
                (END "b")
                (END "a"))))
  (loop for (operator operand) in stream
     do (case operator
          ('START
           (let ((element (list operand)))
             (if stack
                 (progn
                   (setcdr (elt stack 0) (list element))
                   (push element stack))
                 (setq stack (list element))
                 (setq result stack))))
          ('TEXT
           (setcdr (elt stack 0) (list operand)))
          ('END
           (pop stack)))) result)