Can you guess what it does? (Improvement suggestions are welcome ofc)

beginner.clj

(fn mysteryFunction [s]
    ((fn append [mySeq acc] 
	        (let [beginning (butlast mySeq)]
        	        (if (= beginning nil) 
          		        (conj acc (first mySeq))
          		        (append beginning (conj acc (last mySeq)))
                        )
                )
    ) s [])
)

(some obvious Clojure.core functions were not allowed for this exercise ;-)).