[m-rev.] for review: Correct status of partial instantiation support

[Julien Fischer]

On Wed, 12 Mar 2014, Zoltan Somogyi wrote:

> On Wed, 12 Mar 2014 14:49:59 +1100 (EST), Julien Fischer <jfischer at opturion.com> wrote:
>> I suspect that specific code only "works" in those grades more by luck
>> than design.  In general, it does not work.
>
> That is correct, but I think Paul needs to know *why*.
>
> The real reason why partially instantiated data structures do not work in general
> is the lack of alias tracking in the mode system.

Arguably, this shouldn't come as a surprise to anyone.  It is the very
first thing listed in the LIMITATIONS file.

     * We do not allow definite aliasing in the mode system.
       Without this, partially instantiated modes are unusable,
       and so are nested unique modes :-(

Cheers,
Julien.