Re: Lyrics break estimation of vertical spacing

[Boris Shingarov]

> The skylines are used as part of VerticalAlignment, to figure out how
 > far apart the children of VerticalAlignment (ie. VerticalAxisGroups for
 > staves, lyrics, etc) need to be. Once we compute the translations of the
 > children of VerticalAlignment, we can compute the height of
 > VerticalAlignment (and thus the height of System).
 
I understand that this is the general idea how it is supposed to be.
What happens in reality, is that pure_height() of the VerticalAxisGroup,
calling ly:hara-kiri-group-spanner::y-extent in Scheme land,
calling back Hara_kiri_group_spanner::pure_height(), ultimately
ends up in Axis_group_interface::cached_pure_height().
That function, unite()'s the begin_of_line_pure_height() and
the rest_of_line_pure_height(), which are simply measured from
the middle line of the staff, no translations.
 
This is the exact point where I am most confused -- what is the
supposed result of these translations?  So far in the debugger
I only see values relative to the middle line; what is the idea what
these translations are supposed to be?