[Groff] a version of .so that is relative to .so'ing file?

[Dorai Sitaram]

The pathname of an .so argument is resolved
relative to the current working directory (i.e., the
directory in which groff is called).  Is there a way to
get an .so variant, where the .so argument's pathname
is resolved relative to the directory of the file in
which the .so occurs?

(Somewhat like how HTML resolves the filenames in
<a href=...>.)

This would let groff be callable on the same input
file from various working directories with the
same result.