Interesting issue.  Was also not fixed in bipt's wip-elisp branch.

Attached is a patch that attempts to fix this, and some related
issues, both for nil and for t.

The strategy is:

1. All Elisp functions we implement in terms of Scheme must be written
   to special-handle #nil and #t if they accept symbols as arguments.
   (Examples: symbolp, symbol-name, set, makunbound)

2. The Elisp function 'intern' must special-handle "nil" and "t".

This patch, to realize nr. 1, turns #nil and #t into the Scheme
symbols 'nil and 't internally in some situations, but makes sure
not to return those symbols, which are of course not equivalent to 
the #nil and #t values.

The Elisp reader already turns nil and t into #nil and #t, but we
should probably also write a custom Elisp writer that writes them
back as nil and t.  I see you have a separate bug report open for
that issue (https://bugs.gnu.org/39220) and I think we can keep the
two issues separate so no attempt to touch on that in this patch.


- Taylan