nconc with (back)quote'd list constants

Google says: "Avoid nconc!"

http://google-styleguide.googlecode.com/svn/trunk/lispguide.xml#Avoid_NCONC

They recommend to use "mappend" instead, which you could naively define this way (naive because it's O(n^2)!):

(defun mappend (fn list "ul")
  "Append the results of calling fn on each element of list.
  Like mapcan, but uses append instead of nconc."
  (foldl1 append (mapcar fn list)))

(defun foldl1 (function list "ul")
  (if (null list)
      (error "foldl1: empty list")
      (foldl function (car list) (cdr list))))

(defun foldl (function initial list "ugl")
  (let ((result initial))
    (foreach mapc element list
      (setq result (function result element)))
    result))

If you don't see the danger, try this:

ILS-> (mappend (lambda (x) '(t)) '(1 2 3))
(t t t)
ILS-> (mapcan (lambda (x) '(t)) '(1 2 3))

The mappend version works correctly, but the mapcan version sends the interpreter into an infinite loop! 

Explanation: mapcan applies the lambda to 1, and nconcs the result--the constant '(t) list returned by the lambda--to itself, resulting in a circular list of length one, where the car is t and the cdr is its own car.  The next iteration, mapcan applies the lambda to 2, only this time nconc never returns, because it's too busy trying to find the end of an endless list....

I guess I'll have to write my own (foreach mappend....) to use in place of the more dangerous (foreach mapcan...).