[plt-scheme] Requiring SRFIs

Something I did not expect:

Welcome to MzScheme v4.0.1 [3m], Copyright (c) 2004-2008 PLT Scheme Inc.
> (require srfi/48)
> (format "~h")
format: expects argument of type <pattern-string (tag ~h not
allowed)>; given "~h"

 === context ===
/home/henk/local/src/mz-4.0.1/collects/scheme/private/misc.ss:68:7

It looks like I'm not getting the new definition of format. Prefixing
the import solves the problem, but I'm not sure why it is behaving as
above. I would have expected it to either throw an error or shadow the
old definition with the new.

    Henk