Marcel, Its impossible to tell what your lists (sgl, bln, el1) originally contained but the following function will flatten any level of complexity to a single level list (a flat list) (defun flatlist (lst) ;;D. C. Broad, Jr. (cond ((null lst) nil) ((listp (car lst)) (append (flatlist (car lst)) (flatlist (cdr lst)))) (t (cons (car lst) (flatlist (cdr lst)))))) ;|Example execution: $ (flatlist '(((1 2 (3))(4 5))(((6))7)8)) (1 2 3 4 5 6 7 8) |; _$