> \boxXX=
\hbox(0.0+0.0)x60.0
.\vbox(0.0+0.0)x10.0
..\hbox(0.0+0.0)x10.0
..\glue 0.0 plus 1.0fil
.\vbox(0.0+0.0)x20.0
..\hbox(0.0+0.0)x20.0
..\glue 0.0 plus 1.0fil
.\vbox(0.0+0.0)x30.0
..\hbox(0.0+0.0)x30.0
..\glue 0.0 plus 1.0fil
> \cals@spanq@heights=macro:
->{0.0pt}.
%
> \boxXX=
\hbox(0.0+0.0)x60.0
.\vbox(0.0+0.0)x10.0
..\hbox(0.0+0.0)x10.0
..\glue 0.0 plus 1.0fil
.\vbox(0.0+0.0)x20.0
..\hbox(0.0+0.0)x20.0
..\glue 0.0 plus 1.0fil
.\vbox(0.0+0.0)x30.0
..\hbox(0.0+0.0)x30.0
..\glue 0.0 plus 1.0fil
> \cals@spanq@heights=macro:
->{40.0pt}.
%
> \boxXX=
\hbox(0.0+0.0)x0.0
> \cals@spanq@heights=macro:
->.
> 60.0pt.
> 90.0pt.
%
% Bug
%
> 60.0pt.