\def\cals@cs@width{1pt}
\def\cals@framecs@width{2pt}
\def\cals@bgcolor{black}

%
% Normal use
%
\cals@csrow@begin
\def\rborder{4pt}
\cals@csrow@nextcell{100pt}{3pt}\rborder{green}
\def\rborder{6pt}
\cals@csrow@nextcell{110pt}{5pt}\rborder{red}
\cals@csrow@end
\showbox\cals@current@cs

%
% What happens if we have no columns (should never happen)
%
\cals@csrow@begin
\cals@csrow@end
\showbox\cals@current@cs

%
% Defaults are used
%
\cals@csrow@begin
\cals@csrow@nextcell{120pt}\relax\relax\relax
\cals@csrow@nextcell{130pt}\relax\relax\relax
\cals@csrow@end
\showbox\cals@current@cs

%
% test: left frame is bigger as the user-specified width:
% we should not use the maximal width, but drop the default.
% (The first run is not for the test, but for comparation.)
%
\def\rborder{4pt}
\def\cals@framecs@width{2pt}
\cals@csrow@begin
\cals@csrow@nextcell{100pt}{3pt}\rborder{}
\cals@csrow@end
\showbox\cals@current@cs
\def\cals@framecs@width{10pt}
\cals@csrow@begin
\cals@csrow@nextcell{100pt}{3pt}\rborder{}
\cals@csrow@end
\showbox\cals@current@cs

%
% Bug hunting (a space)
%
\setbox\cals@current@cs=\hbox{}
\let\cals@lastWidth=\relax
\cals@csrow@nextcell\relax\cals@framecs@width\relax\relax
\showbox\cals@current@cs