Understanding the processing model

[Martin Hosken]

Is there any documentation that explains more of the internal workings of 
lilypond. I'm looking for something that explains how music events get to 
engravers and then what happens afterwards with the layout events. 'When' are 
stencils executed? Is it possible to write an engraver in scm?

I am managing to get somewhere with writing scheme code to do what I want, but 
the effects are usually more through luck than judgement: I write something 
expecting something to happen and it doesn't so I have to write it a different 
way. I think if I understood the overall processing of events better from music 
events through to output events and when things execute, I could do a whole lot 
better.

TIA,
Yours,
Martin