[gclist] other GC services

[Nick Barnes]

> The big problem would be debugging. A bug in a copy procedure would
> manifest itself at arbitrary points in the program; it might be very
> hard to find. Only experience would tell us how the hazards and
> advantages trade off.

So you have a mode in which specialized copy procedures are turned
off, or a way of expressing the semantics which need to be preserved
and an automated before/after check, or required proof certificates
(see Necula & Lee on self-certifying code).

> I can't recall ever seeing anything like this.

Hash-consing is like this.

Nick B