[texhax] Weirder and weirder

[Philip Taylor (Webmaster, Ret'd)]

Uwe Lueck wrote:

 > I think the OP is still wondering at which .tex lines
 > TeX replaces \nullfont by the font acting in typesetting.
 > I didn't remember well myself, but the OP's .log indicates
 > that the \textfont's (TeXbook p. 153) are chosen at the
 > first $, which triggers LaTeX's \check at mathfonts.

Well done, Uwe : you have saved me a great deal of work
in having to track that down !  I promised Michael I would
investigate today, but did not yet feel awake enough to start.

Michael, I wonder whether this provides the hook you need :
once the various \textfonts have been set, is the probability
sufficiently high that one of these will also be the default
text font that you could make use of it in your em-based
calculations ?  If not, then I think that you could still
make use of my earlier suggesting (deferred definition of
\em) by wrapping all the definitions concerned in one large
macro which is then called \AtBeginDocument.

** Phil.