[Small-cl-src] Computed features expressions

[Helmut Eller]

;; Here's a little trick I just discovered.  Dunno how portable it is.
;; 
;; I often would like to write code like: 
;;
;;  (if <complicated-test>  
;;      (defun foo () <version1>) 
;;      (defun foo () <version2>))
;;
;; The drawback of this idiom is that FOO is no longer at toplevel.
;; If we write
;;
;;   #+<complicated-test>
;;   (defun foo () <version1>)
;;   #-<complicated-test>
;;   (defun foo () <version2>)
;;
;; FOO would be at toplevel, but <complicated-test> can only contain
;; simple feature expressions.
;;
;; But if we write
;;
;;   #+#.(if <complicated-test> '(and) '(or))
;;   (defun foo () <version1>)
;;   #-#.(if <complicated-test> '(and) '(or))
;;   (defun foo () <version2>)
;;
;; we get the advantages of both variants.

;; Define ASSQ unless it is already defined:

#+#.(cl:if (cl:fboundp 'cl-user::assq) '(or) '(and))
(defun assq (item list)
  (assoc item list :test #'eq))