Meaning of X-extent and Y-extent

[Noeck]

Hi,

I tried to help in the thread "TabStaff arpeggio problem when first
thing on line" but I realized that I do not understand X- and Y-extends.

Is there more to find than this
http://lilypond.org/doc/v2.18/Documentation/notation/aligning-objects
and
http://lilypond.1069038.n5.nabble.com/accidental-spacing-multiple-signs-X-extent-and-ly-accidental-interface-width-td50448.html
?

How are numbers of the "wrong" sign treated?
X-extent = #'(2 . -2) % instead of (-2 . 2) for instance
Is the object width then -4 staff spaces or 0 or sth. else?
I see some unpredictable behaviour. For instance,
(0 . -0.0001) is different from (0 . 0) but not from (0 . -10). (0 . 0)
leaves some space for the object, the negative values seem to collapse
it completely. How does that happen?

Are these distances like rectangles around the reference point?
And no such rectangles may overlap?

Is there a way to print them (like "show control points" in Frescobaldi
or "annotate-spacing" in the paper block)?

Cheers,
Joram