omit `deleted-signal` & correct docs (#13)

txexpr/base.rkt

@@ -261,8 +261,7 @@
               x))))
 
 ;; function to split tag out of txexpr
-(define deleted-signal (gensym))
-(define+provide+safe (splitf-txexpr tx pred [proc (λ (x) deleted-signal)])
+(define+provide+safe (splitf-txexpr tx pred [proc (λ (x) #f)])
   ((txexpr? procedure?) (procedure?) . ->* . (values txexpr? txexpr-elements?))
   (unless (txexpr? tx)
     (raise-argument-error 'splitf-txexpr "txexpr?" tx))
@@ -273,8 +272,7 @@
        (set! matches (cons x matches))
        (proc x)]
       [(? txexpr?) (let-values ([(tag attrs elements) (txexpr->values x)]) 
-                     (txexpr-unsafe tag attrs (filter-not (λ (e) (eq? e deleted-signal))
-                                                          (map extract! elements))))]
+                     (txexpr-unsafe tag attrs (filter values (map extract! elements))))]
       [_ x]))
   (define tx-extracted (extract! tx)) ;; do this first to fill matches
   (values tx-extracted (reverse matches)))

txexpr/scribblings/txexpr.scrbl

@@ -473,7 +473,7 @@ In practice, most @racket[_txexpr-element]s are strings. But it's unwise to pass
 (splitf-txexpr
 [tx txexpr?]
 [pred procedure?]
-[replace-proc procedure? (λ (x) null)])
+[replace-proc procedure? (λ (x) #f)])
 (values txexpr? (listof txexpr-element?))]
 Recursively descend through @racket[_txexpr] and extract all elements that match @racket[_pred]. Returns two values: a @racket[_txexpr] with the matching elements removed, and the list of matching elements. Sort of esoteric, but I've needed it more than once, so here it is.