[Sorry, Alex, for sending this to you twice - I accidentally made my
 last message a narrow, rather than wide, reply.]

Alex Branham <alex.branham@gmail.com> writes:

> Thanks for the feedback, everyone.

Thanks for working on this.

> Here's a patch that implements `flatten-tree' which always returns a
> list and recurses into conses.

Given Emacs' recursive limitations, wouldn't an iterative implementation
be better?  For instance, the following currently blows
max-specpdl-size:

  (length (flatten-tree (make-list 800 nil)))

How about something like the following?