--mulisp D:prlg18.lsp

(DEFUN SOLVE (lst)
  (COND
    ( (NULL lst) NIL )
    ( (ATOM (CAR lst)) (CONS (CAR lst) (SOLVE (CDR lst))) )
    ( 1 (APPEND (SOLVE (CAR lst)) (SOLVE (CDR lst))) )
  ) 
)

(SOLVE '(1 (2 3) 4))
(SOLVE '(a (b c) a b (c (c)))))