%
% Empty
%
> \boxXX=
\hbox(0.0+0.0)x0.0
%
% One element
%
> \boxXX=
\hbox(3.0+3.0)x100.0
.\glue -1.5
.\rule(3.0+3.0)x104.0
.\glue -2.5
%
% Several elements
%
> \boxXX=
\hbox(4.0+4.0)x303.0
.\glue -1.0
.\rule(3.0+3.0)x103.0
.\glue -2.0
.\glue -100.0
.\glue -2.0
.\rule(4.0+4.0)x104.0
.\glue -2.0
.\glue -1.0
.\rule(0.5+0.5)x104.0
.\glue -2.0
.\glue 102.0