Re: Texinfo TeX accepts LaTeX math, how?

[Gavin Smith]

On Mon, Oct 26, 2020 at 07:18:01PM +0100, Patrice Dumas wrote:
> > Another option is to use htlatex instead (if there is such a program),
> > but that could be problematic if there are any important usages of
> > plain TeX that don't work with LaTeX.
> 
> I have no idea on that.  It is not clear to me to what extent LaTeX is a
> superset of TeX or something different.  If it is a superset, indeed,
> LaTeX tools could always be used, which could be simpler in some cases.

Here's the best information I've found so far.  This is a package
to use plain TeX in LaTeX:

https://ctan.org/pkg/plain-ltx?lang=en

> Inside the environment, not everything is as Knuth intended
> (some Plain commands are suppressed, some are faked, and some basic
> mechanisms—such as font selection and the output routine—remain
> as in LATEX). Thus “simple” documents will run, but elaborate
> macros defined in the document have a chance of failing.

The only macro that I can see that looks relevant for math from
looking at the source of that package is \eqalign.