[tex-eplain] bug in eplain 3.8

[Karl Berry]

    \providecommand

    fails with the error message

    !!! Undefined control sequence.
    <recently read> \@optionalusercs

(I wonder how you got three ! characters there.)

I'm releasing eplain 3.9 shortly with what I hope is the fix for this. I
believe it's one line in eplain.tex (same in btxmac.tex):

-     \let\cs=\@continuenewcommand  % undefined, so we'll define it
+     \let\cs=\@btxcontinuenewcommand  % undefined, so we'll define it


On another front, for everyone: I couldn't reproduce various
double-column bug reports, although no changes have been made in this
area for a while. If double-column bugs continue to crop up (I don't
doubt they will), please report them.


Thanks,
Karl

P.S. Rodolfo, I did look at the PDF outline master/child example you
sent me, but I failed to extract a feature to add to Eplain. Sorry.
I did add the bookmark example from my email to you
(https://tug.org/pipermail/tex-eplain/2016/000911.html) to the manual.
If you think there's something more I should add to the code or doc, let
me know.