[plt-scheme] srfi 1 impl: delete-duplicates!, append! - bugs?

Hi, all!

I can not understand, is these bugs or features?

Welcome to MzScheme version 301, Copyright (c) 2004-2005 PLT Scheme Inc.
 > (require (lib "1.ss" "srfi"))
 > (define a '((1 . 2) (2 . 3) (2 . 3)))
 > a
((1 . 2) (2 . 3) (2 . 3))
 > (delete-duplicates! a)
((1 . 2) (2 . 3))
 > a
((1 . 2) (2 . 3) (2 . 3))
 >

And

Welcome to MzScheme version 301, Copyright (c) 2004-2005 PLT Scheme Inc.
 > (require (lib "1.ss" "srfi"))
 > (define b '())
 > b
()
 > (append! b 1)
1
 > b
()
 >

Igor.