Slurs after line break - comments affect layout?

[Risto Vääräniemi]

Dear All,

I have a piece of music with many slurs and sometimes I need to make a
system break during a slur. With the current key (f minor) the default
results are usually not that pleasing to the eye.

Luckily, I found a Scheme thingy on the mailing list for tweaking the
broken slur at the beginning of a system.
http://lists.gnu.org/archive/html/lilypond-user/2007-08/msg00539.html

There's something odd in that, though. It seems that comments and
spacing affects the resulting slur, which sounds weird.

%%%%
\version "2.11.39"

\include "suomi.ly"

\paper
{
    indent = 0\mm
    ragged-right = ##t
}

#(define (my-callback grob)
    (let* (
        ; have we been split?
        (orig (ly:grob-original grob))
            ; if yes, get the split pieces (our siblings)
            (siblings (if (ly:grob? orig)
            (ly:spanner-broken-into orig) '() )))
            (if (and (>= (length siblings) 2)
                       (eq? (car (last-pair siblings)) grob))
               (ly:grob-set-property! grob 'control-points
               '((4 . 3) (5 . 4.8) (10 . 5) (11 . 3.5))))))
%((0.3 . 1.8) (-0.3 . 3) (-2 . 3.5) (-3 . 3))
%((4 . 9) (6 . 13) (17 . 14) (21 . 12))
%((4 . 3) (5 . 4.8) (10 . 5) (11 . 3.5))

\relative c''
{
    \clef treble \time 4/4 \key f \minor

    %\once \rideover Srul #'sotipions = #'(0 . 1.0)

    g1 \once \override Slur #'after-line-breaking = #my-callback
    g2^( g4 g \break
    g2) g2
    g1

    % foobar
}
%%%%

The broken slur reverts to default shape if I remove the line break
above the % foobar, remove the %\once ... line, remove the \include
"suomi.ly" line or remove the commented list of numbers below the
Scheme function. The stuff on the %\once line probably doesn't have
any effect per se because messing up the syntax didn't change the
behaviour.

Question one:
If the Scheme is the culprit is there a way to fix it?

Question two:
Can I pass the control points as an argument?

Question three:
Is there another way to tweak the shape of the broken slur?

-Risto

default_slur.png
Description: PNG image

tweaked_slur.png
Description: PNG image