\def\defaultwidth{1pt}

%
% Empty
%
\def\rsI{}
\def\rsII{}
\def\mkbox{\setbox0=\hbox{\cals@rs@sofar@reset \cals@rs@joinTwo\defaultwidth\rsI\rsII \cals@rs@sofar@end}}
\mkbox
\showbox0

%
% One element
%
\cals@rs@pack\rsI{100pt}{2pt}{4pt}{6pt}
\cals@rs@pack\rsII{100pt}{3pt}{5pt}{6pt}
\edef\rsI{{\rsI}}
\edef\rsII{{\rsII}}
\mkbox
\showbox0

%
% Several elements, with no-line and default
%
\cals@rs@pack\frI{100pt}{2pt}{4pt}{6pt}
\cals@rs@pack\frII{101pt}{2pt}{4pt}\relax
\cals@rs@pack\frIII{102pt}{2pt}{4pt}{0pt}
\edef\rsI{{\frI}{\frII}{\frIII}}
\cals@rs@pack\frI{100pt}{4pt}{4pt}{8pt}
\cals@rs@pack\frII{101pt}{2pt}{4pt}\relax
\cals@rs@pack\frIII{102pt}{3pt}{5pt}{0pt}
\edef\rsII{{\frI}{\frII}{\frIII}}
\mkbox
\showbox0