The alignments, and whether borders are to be employed, are decided from the ‘(|c|@{...}l|r|...)’ like parameter. A border is set if the parameter request a drawing of a line.
<..init conds for @mkpream..>
\def\Clr{#2}\a:VBorder
\HAssign\ar:cnt=0
\let\HAlign=\empty
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