column spanning major headers in liststore/treeview?

[george young <gry ll mit edu>]

[pygtk-1.99.17 , gtk+-2.2.4, python-2.3.3]

I have a ListStore in a TreeView.  There are various numbers
of columns with various column headers, determined at runtime, and 
changing throughout the session.

There are three groups of columns, and I would like to put a major
header centered over each of two groups, spanning the whole group:

                INSTRUCTIONS            MEASUREMENTS
wafers     temp  speed  ramp-rate    thickness  stddev
1          350   200    12.3         19         0.33
2          365   230    12.3         22         0.79

Naturally, I would like the major headers to keep themselves
centered over their children as their subsidiary columns change 
in width.

Any suggestions?

-- George Young

-- 
"Are the gods not just?"  "Oh no, child.
What would become of us if they were?" (CSL)