our take on the issue is available at:
http://dwim.hu/darcsweb/darcsweb.cgi?r=HEAD%20hu.dwim.def;a=headblob;f=/test...
it's basically a macro that defines both a call-with-... function and a with-... macro.
the body is written as if it was the body of the call-with... function, but there's a lexically visible function similarly to (call-next-method) to invoke the user given forms.