system position, extents, space, padding

[Nicolas Sceaux]

Hi,

I've hacked a very rough page breaking scheme, but I'd like to improve
it (it only takes care of padding, not space, and is clueless about how
extent really work). Thus, I've a few questions about how are computed
systems position on a page, in particular when it comes to consider
properties like between system space, which include the extent of staves
(sic).

1) The system position as computed in `optimal-page-breaks' ('position
   property of a page) is the top of the Y-extent arrow?

     -                          ^
/ ---|---                       |
| ---|---                       |
| ---|---  ^                    |
| ---O---  |                    |
| -------  |                    |
|          |refpoint-Y-extent   |Y-extent  
| -------  |                    |
| -------  |                    |
| ---|---  v   ^                |
| ---|---      |                |
\ ---|---      |                |
    -O-        |                v
               |                ^
               |                |next-padding
               |next-space      v
               |
               |
     -         |                ^
/ ---|---      v                |
| ---|---                       |
| ---|---  ^                    |
| ---O---  |                    |
| -------  |                    |
|          |refpoint-Y-extent   |Y-extent  
| -------  |                    |
| -------  |                    |
| ---|---  v                    |
| ---|---                       |
\ ---|---                       |
    -O-                         v
                                

2) How to compute the 2nd system position, given the two system Y-extent
   and refpoint-Y-extent, and the first system next-space and position?
   (disregarding next-padding, supposed too short here). I've been lost
   when reading ly:solve-spring-rod-problem, and I don't really
   understand what the two numbers are in extents.

nicolas