Distance between a bar line and the following note

[Xavier Scheuer]

Hello,

I'd like to align a RehearsalMark on an "anchor" that is _exactly in
the middle_ between a Barline and the following note (first note of the
measure).

I read in the Internals Reference manual that for the "Barline" object
(IR 3.1 All layout objects > 3.1.11 BarLine)
the value for 'space-alist contains

  (first-note fixed-space . 1.3)
  (next-note semi-fixed-space . 0.9)

What's the difference between "first-note" and "next-note"?
Which one refers to the first note of the measure?

When it says "semi-fixed-space" I understand it is stretcheable
(horizontally).  That seems perfectly plausible since we can have, for
example, 4 or 8 times the *same* measure on a line.  Thus the distance
between notes or between BarLine and the first note can be either
stretched or compressed.

Is there a minimal and maximal value to a "semi-fixed-space" ?
Would it be possible to determine "on-the-fly" this distance in order
to move RehearsalMark #'break-align-anchor to half this distance?

Many thanks in advance.

Cheers,
Xavier

-- 
Xavier Scheuer <address@hidden>