How to revert 'metronomeMarkFormatter'

[Thomas Morley]

Hi,

I'm currently working on improving 'metronomeMarkFormatter', related
to issue 3096
http://code.google.com/p/lilypond/issues/detail?id=3096

I stumbled about an issue I don't know how to handle.
If another metronomeMarkFormatter is used, it should be possible to
revert it as usual, i.e.:
\set Score.metronomeMarkFormatter = #<what-ever>
and revert with
\unset Score.metronomeMarkFormatter

Doesn't work.
Below a small example and the log-output.

% Example

\version "2.17.28"

#(define (color-format-metronome-markup event context)
  (make-with-color-markup red
    (format-metronome-markup event context)))

\relative c' {
    c1
    %% default
    \set Score.metronomeMarkFormatter = #format-metronome-markup
    \tempo "xx"
    d
    %% changed
    \set Score.metronomeMarkFormatter = #color-format-metronome-markup
    \tempo "yy"
    e
    %% revert??
    \unset Score.metronomeMarkFormatter
    %% explicit writing a new/old one works. Change to:
    %\set Score.metronomeMarkFormatter = #format-metronome-markup
    \tempo "zz"
    f
}


%% log
GNU LilyPond 2.17.28
Processing `atest-22.ly'
Parsing...
Interpreting 
music.../home/me/lilydevel/usr/share/lilypond/current/scm/lily-library.scm:242:5:
In procedure apply in expression (process-procedure book paper ...):
/home/me/lilydevel/usr/share/lilypond/current/scm/lily-library.scm:242:5:
Wrong type argument in position 1: ()


What to do?

Thanks,
  Harm