[tex-eplain] btxmac.tex causes Runaway definition and Forbidden control sequence on compile.

[Karl Berry]

    > \beginpackages
    > \usepackage{url}

    I don't know about this. Is this defined in (plain) TeX when using
    eplain? 

\beginpackages, \usepackage, etc. are defined in eplain. plain.tex knows
nothing about them (as one would expected).

Rest is background ...

The LaTeX package stuff is only possible because the LaTeX group
provides a file miniltx.tex that has enough of LaTeX to support the
graphics, color, and url packages, and a few others (not hyperref).
Thus, independent of eplain, this works, for example:

\input miniltx
\input graphicx
\includegraphics[...options...]{someimage}

which can be quite useful sometimes, when the simple scaling provided by
pdftex is not enough.

Packages known to work with Eplain:
https://tug.org/texinfohtml/eplain.html#Packages-known-to-work

Packages known not to work with Eplain:
https://tug.org/texinfohtml/eplain.html#Packages-known-not-to-work

I'm not sure if Eplain provides a little wrapper code for some
packages. I suspect not, and the list is the same as for miniltx.

BTW, this section explains why we added \beginpackages to Eplain at all:
https://tug.org/texinfohtml/eplain.html#Environment-for-loading-packages

Best,
Karl