Texinfo TeX accepts LaTeX math, how?

[Patrice Dumas]

Hello,

Surprisingly (for me), the @displaymath example in the Texinfo manual
works when producing a pdf using texi2dvi.  However, it uses \frac{}{},
which is not not a plain TeX construct, as far as I know.  How is that
possible?  If the math in Texinfo is "LaTeX math" it is much simpler for
writers, as "plain TeX math" is less known that "LaTeX math" but also
for math rendering in HTML, as it means that latex2html can be used
without trouble and htlatex can be used instead of httex.

If it is not the same as LaTeX math, but a superset of plain TeX math,
could you tell me how I can modify the TeX file produced by tex4ht.pm
for httex such that there is support of the same constructs in math
rendering through tex4ht?  Right now, \frac appears as undefined when
processing with httex.

-- 
Pat