ANSI Common Lisp   6 Iteration   6.1 The LOOP Facility   6.1.3 Value Accumulation Clauses

6.1.3.2 Examples of APPEND and NCONC clauses

;; 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))