(TOP
  (S
    (NP
      (PRP I))
    (VP
      (VB mind)
      (NP
        (PRP$ their)
        (NN smoking)
        (NNS cigars))
      (PP
        (IN in)
        (NP (PRP$ my)
        (NN car))))
    (. .)))


Transform -->

(:tag TOP :chunk
  (:tag S :chunk
    [(:tag NP :chunk (:tag PRP :chunk I))
     (:tag VP :chunk [(:tag VB :chunk mind)
                      (:tag NP :chunk [(:tag PRP$ :chunk their)
                                       (:tag NN :chunk smoking)
                                       (:tag NNS :chunk cigars)])
                      (:tag PP :chunk [(:tag IN :chunk in)
                                       (:tag NP :chunk (:tag PRP$ :chunk my))
                                       (:tag NN :chunk car)])])
     (:tag . :chunk .)]))

(def a '(TOP
     (S
      (NP
       (PRP I))
      (VP
       (VB mind)
       (NP
        (PRP$ their)
        (NN smoking)
        (NNS cigars))
       (PP
        (IN in)
        (NP (PRP$ my)
        (NN car))))
      (. .))))

(defn tr [to-parse]
  (cond (symbol? to-parse) to-parse
    (seq to-parse) (let [[tag & body] to-parse]
             `(:tag ~tag :chunk ~(if (> (count body) 1)
                           (map tr body)
                           (tr (first body)))))))

(tr a)
(:tag
 TOP
 :chunk
 (:tag
  S
  :chunk
  ((:tag NP :chunk (:tag PRP :chunk I))
   (:tag
    VP
    :chunk
    ((:tag VB :chunk mind)
     (:tag
      NP
      :chunk
      ((:tag PRP$ :chunk their)
       (:tag NN :chunk smoking)
       (:tag NNS :chunk cigars)))
     (:tag
      PP
      :chunk
      ((:tag IN :chunk in)
       (:tag
        NP
        :chunk
        ((:tag PRP$ :chunk my) (:tag NN :chunk car)))))))
   (:tag . :chunk .))))