- Hacked append-reverse! so that it won't blow up on improper lists.

- Hacked delete so that it shares storage when possible -- it doesn't
  always copy the entire list. For example, (delete lis (lambda (x) #f))
  returns lis, allocating no new list structure. (It allocates stack frames,
  of course.)

big/big-util.scm

@@ -170,23 +170,34 @@
 (define (delq thing list)
   (delete (lambda (x) (eq? x thing)) list))
 
-(define (delete pred in-list)
-  (let loop ((list in-list) (res '()))
-    (cond ((null? list)
-	   in-list)
-	  ((pred (car list))
-	   (append-reverse! res (cdr list)))
-	  (else
-	   (loop (cdr list) (cons (car list) res))))))
-
 (define (append-reverse! l1 l2)
   (let loop ((list l1) (res l2))
-    (cond ((null? list)
-	   res)
-	  (else
-	   (let ((next (cdr list)))
-	     (set-cdr! list res)
-	     (loop next list))))))
-
+    (if (pair? list)
+	(let ((next (cdr list)))
+	  (set-cdr! list res)
+	  (loop next list))
+	res)))
 
+;; This DELETE shares the longest tail of L that has no deleted elements.
+;; If we had multi-continuation calls, this could be made more efficient.
 
+(define (delete pred l)
+  (call-with-values (lambda ()
+		      ;; (recur l) returns L with (pred x) values deleted.
+		      ;; It also returns a flag NO-DEL? if the returned value
+		      ;; is EQ? to L, i.e. if it didn't delete anything.
+		      (let recur ((l l))
+			(if (pair? l)
+			    (let ((x (car l))
+				  (tl (cdr l)))
+			      (if (pred x)
+				  (call-with-values (lambda () (recur tl))
+				    (lambda (ans no-del?)
+				      (values ans #f))) ; Deleted X.
+				  (call-with-values (lambda () (recur tl))
+				    (lambda (ans no-del?)
+				      (if no-del?
+					  (values l #t)
+					  (values (cons x ans) #f))))))
+			    (values '() #t))))
+    (lambda (ans no-del?) ans)))