/Line

Why \centerline are defined also for latex and non-sty env, but \line not.

Check \a:leftline and \b:leftline before making any change in \line.

<..plain+ env..>+
 \pend:def\line{\ifx \EndPicture\:UnDef \hbox{\ht:everypar{}\a:line}\fi}
 -_-_-

The ‘\hbox’ on the group is required for preserving vertical modes.

<..config plain utilities..>+
 \NewConfigure{line}{1}
 -_-_-