<..latex ltplain..>
<.plain,ltplain obeylines,oalign.>
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plain.tex and latex.
The following is assumed to be within a group.
<..plain,ltplain obeylines,oalign..>
\def\:temp{\o:obeylines:
\let\obeylines=\o:obeylines:
\a:obyln \global\let\x:obln=\end:obeylines \aftergroup\x:obln
\def\:temp{%
\ifx\:tempa\par \ht:everypar{\ht:everypar{\b:obyln}}%
\else \ht:everypar{\b:obyln}\fi}%
\futurelet\:tempa\:temp }
\HLet\obeylines=\:temp
\NewConfigure{obeylines}[3]{\c:def\a:obyln{#1}\c:def\b:obyln{#3}%
\c:def\end:obeylines{#2}}
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[more]