[gtkmm] Q: how to centre in a VBox?

["D. R. Evans" <N7DR arrisi com>]

I'm sure this is utterly trivial... but after several hours of trying, I'm 
giving up and asking for help.

I have several HBoxes, which I would like to pack into a VBox that is 
displayed as a page in a Notebook.

How do I pack things so that when the Notebook page displays, the HBoxes are 
vertically centred (either exactly or to a reasonable approximation) on the 
page? So that the page looks like:

-------------------------------
|                             |
|                             |
|        hbox here            |
|        hbox here            |
|        hbox here            |
|                             |
|                             |
-------------------------------

  Doc