how to use diagram-viewer and the diagram pkg

I would like to try out the diagram-viewer app and the diagram pkg.
I am using Tcl/Tk 9.0b2 on Linux.

Given file /tmp/a.dia (copied from the tclwiki):

box A
arrow "" 40
circle B
direction SE

I tried this at the command line:

$ wish9 ~/opt/tcl9/bin/diagram-viewer show /tmp/a.dia
Creating safe environment, please wait...
/home/mark/opt/tcl9/bin/diagram-viewer: can't find package diagram
$ wish9
% package require diagram
1

As you can see, diagram-viewer can't find the diagram package even though
it is there.

Nor can I find any other examples of the diagram language nor what file
extension one should use.