From: John Williams
Subject: Re: Weak binding?
Message-ID: <41cs6g$>
>The question is: Is there a way to implement this portably
>in common-lisp? My gut feel for this is no, otherwise I guess
>I'd be using it and not posting here. I suppose various
>implementations might offer some access to underlying gc controls.

Poplog Common Lisp allows you to create hash-tables where a key-value
binding is gc'd if either the key or value becomes garbage (you can
specify which).