;; Use APPEND to concatenate some sublists.
  (loop for x in '((a) (b) ((c)))
        append x)
⇒  (A B (C))

;; NCONC some sublists together.  Note that only lists made by the
;; call to LIST are modified.
  (loop for i upfrom 0 
        as x in '(a b (c))
        nconc (if (evenp i) (list x) nil))
⇒  (A (C))