>Perhaps it is time we went the extra mile and added  a Gardner-style
>nested-allpass opcode, since it does look like a useful building-block.

Yes.  I am currently working on some other stuff but if no one else writes
one before I get to it I'll do it.

>I do wonder if some of that is not deliberately
>obfuscated, like the ancient alchemists used to do!),

That would be industries job.  I suspect the reverbs were derived largely
empirically.  The main goal is to rapidly build up echo density without
generating metallic ringing.

and I think that
>there is plenty of scope for alternative designs.

I have seen other designs based on nested all pass filters.

Bye,
Hans Mikelson