> \boxXX=
\hbox(0.0+0.0)x0.0
> \boxXX=
\hbox(3.0+3.0)x100.0
.\glue -1.0
.\rule(3.0+3.0)x103.0
.\glue -2.0
> \boxXX=
\hbox(3.0+3.0)x403.0
.\glue -1.0
.\rule(3.0+3.0)x203.0
.\glue -2.0
.\glue -1.0
.\rule(0.5+0.5)x104.0
.\glue -2.0
.\glue 102.0