Update to my toy.

Function may return #f rather than cons, which will leave the list it’s building as is. Was needed for filter. Also slows it down a great deal. :[

(define (stillhasnoname lst func)
  ;; Grab pointer to first cons.
  ;; Contents of car will be dropped.
  ;; Contents of cdr will be clobbered. 
  (let ((newlst (list #f))) 
    ;; acc is usually the result of the
    ;; "reduce so far", here it is the
    ;; last pair in newlst. cur is
    ;; current item of consideration
    ;; in the original list. 
    (lfold (lambda (acc cur)
	     ;; func should return either 
	     ;; a new pair to be tacked 
	     ;; onto the end of newlst,
	     ;; or #f which will leave
	     ;; acc as-is. 
	     (let ((tst (func cur)))
	       (if tst
		   (begin (set-cdr! acc tst)
			  (rest acc))
		   acc)))
           newlst ; Accumulator
           lst) ; list we're deriving from
    (rest newlst))) ; car is trash, see above.

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