Re: @displaymath \leqalignno problem for pdf output

[Gavin Smith]

On Fri, Nov 13, 2020 at 09:33:46PM +0000, Gavin Smith wrote:
> On Thu, Nov 12, 2020 at 08:28:51PM +0100, Christopher Dimech wrote:
> > I am seeing problems with using \eqno
> > 
> > For instance, the following fails in html, but works in pdf.
> > 
> > @displaymath
> >    v(t) = {d \over dt} u(t)\eqno{[3.1]}
> > @end displaymath
> 
> I can confirm this; I will look into it.
> 
> In future, please include more detail in your bug reports: the
> exact input you used and what the result was.
> 
> I get the following:
> 
> ! Display math should end with $$.
> <to be read again> 
>                    @def 
> @parseargusing #1#2->@def 
>                           @argtorun {#2}@begingroup @obeylines 
> @spaceisspace...
> l.9 @end
>          displaymath
> ? h
> The `$' that I just saw supposedly matches a previous `$$'.
> So I shall assume that you typed `$$' both times.

I don't really understand this error message but it seems that the
parsing of the '@end displaymath' directive is getting in the way.
According to documentation of TeX, \eqno should be followed immediately
by $$.

I found a fix for this in commit a2cc1f914 by temporarily redefining @end
to exit display math mode straight away.  This works for both the alignment
(which uses \halign) and \eqno.