Understanding and modifying font nodes

[Urs Liska]

I have wrapped around about anything I could think of, but I'm stuck and 
would greatly benefit of some help now.

I'm working on the font interface and will (hopefully soon) present a 
patch that may improve font handling (with the new alternative fonts 
provided by Abraham) significantly. The last big obstacle is modifying 
the add-font procedure in font.scm to an "add-or-replace-font" 
behaviour. The idea is removing the annoying limitation that fonts can 
only be set once and not reset. In order to change anything one has to 
recreate the complete font-tree.

Investigating add-font shows that this limitation is somewhat intended.
In the line when it says
(if (not child)
the procedure is skipped when the font node has already been set, so 
this is what I want to change.

When I try to remove that conditional I *can* reset the fonts, but two 
things happen:
- I get the right font but with the wrong variant (bold italic instead 
of default upright).
- All fonts seem to overwrite the 'roman' variant, regardless of what 
variant has been passed as argument

It boils down that I simply don't understand what's going on inside the 
font nodes and how many of them are processed in the iterations, 
regardless how much debug printing I use to get to grips with it.

Can anybody help me out with modifying this place in font.scm?
Or at least get me back on track.

Best
Urs