Re: @displaymath \leqalignno problem for pdf output

[Christopher Dimech]

> Sent: Thursday, November 12, 2020 at 10:35 PM
> From: "Gavin Smith" <gavinsmith0123@gmail.com>
> To: "Christopher Dimech" <dimech@gmx.com>
> Cc: "help-texinfo gnu" <help-texinfo@gnu.org>
> Subject: Re: @displaymath \leqalignno problem for pdf output
>
> On Thu, Nov 12, 2020 at 08:15:59PM +0100, Christopher Dimech wrote:
> >
> > I have been testing some more and we are getting some serious problems.
> > Have tried the following two constructs
> >
> > ------------------------------------
> >
> > 1. First Test - This works in html, but fails in pdf ($$ $$ are required)
> >
> > @displaymath
> > \leqalignno{
> >    \int v(t)\, dt &= u(t) + C&[3.1]\cr
> >    \int v(t)\, dt &= u(t) + H&[3.2]\cr
> > }
> > @end displaymath
> >
> > ---------------------------------------
>
> This appears to be a bug in the implementation in texinfo.tex but it
> will take me some time to work out what is going on.

That's proper.  I picked up on the subtleties by firing some brutal tests.
Exciting times.  Thank you so very much.

A small thing that can be a quick fix is that @displaymath introduces
excessive vertical spacing when using \displaylines in pdf output.  The
output is good for html.

Test:

Using the Eulerian Time Derivative Operator @math{D_t} and
the Eulerian Time Anti-Derivative Operator @math{D_t^{-1}}, the ground
surface displacement @math{u(t)} can be computed by taking the
Anti-Derivative of the ground surface velocity @math{v(t)}.

@displaymath
\displaylines{
   v(t) = D_t u(t)\cr
   D_t^{-1} v(t) = D_t^{-1} D_t u(t)\cr}
@end displaymath

> There is actually a help message:
>
> ! Missing $$ inserted.
> <to be read again>
>                    @begingroup
> @parseargusing ...>@def @argtorun {#2}@begingroup
>                                                   @obeylines @spaceisspace 
> #...
> l.15 @end
>           displaymath
> ? h
> Displays can use special alignments (like \eqalignno)
> only if nothing but the alignment itself is between $$'s.
>
> ?
>