%plain TeX version of comment package -- much faster!
\let\isafmtname\fmtname\def\fmtname{plain}
\usepackage{comment}
\let\fmtname\isafmtname

\newcommand{\isakeeptag}[1]%
{\includecomment{isadelim#1}\includecomment{isatag#1}\csarg\def{isafold#1}{}}
\newcommand{\isadroptag}[1]%
{\excludecomment{isadelim#1}\excludecomment{isatag#1}\csarg\def{isafold#1}{}}
\newcommand{\isafoldtag}[1]%
{\includecomment{isadelim#1}\excludecomment{isatag#1}\csarg\def{isafold#1}{\isafold{#1}}}

\isakeeptag{ML}
\isakeeptag{document}
\isakeeptag{important}
\isadroptag{invisible}
\isakeeptag{proof}
\isakeeptag{theory}
\isakeeptag{unimportant}
\isakeeptag{visible}
