recons / relist / relist*

util-recons.lisp

;;; 2012/10/27 @nfunato
;;; Probably I first saw them at Xerox PCL about 25 years ago.
;;; I think almost same code can be seen in many places, including CLtL2. 

;;; recons / relist / relist*

(defun recons (x a d)
  (destructuring-bind (xa . xd) x
    (if (and (eql xa a) (eql xd d))
      x
      (cons a d))))

(defun relist-1 (x a d star?)
  (if (null d)
    (if star? a (recons x a d))
    (recons x a (relist-1 (cdr x) (car d) (cdr d) star?))))

(defun relist (x &rest args)
  (if (null args)
    nil
    (relist-1 x (car args) (cdr args) nil)))

(defun relist* (x a &rest d)
  (relist-1 x a d t))