scheme engravers cause "Warnung: Attempting to remove nonexisting listener."

[Reinhold Kainhofer]

If one uses a scheme engraver that instantiates itself for each context, i.e. 
   \consists #(lambda (context) ...)
then using listeners will cause the warning:
   "Warnung: Attempting to remove nonexisting listener."

Minimal test case attached. Output on the console is:
address@hidden:~$ LANGUAGE=C lilypond Issue_RemovingNonExistingListener.ly
GNU LilyPond 2.15.12
Processing `Issue_RemovingNonExistingListener.ly'
Parsing...
Interpreting music... 
warning: Attempting to remove nonexisting listener.
warning: Attempting to remove nonexisting listener.
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `Issue_RemovingNonExistingListener.ps'...
Converting to `./Issue_RemovingNonExistingListener.pdf'...
Success: compilation successfully completed


Cheers,
Reinhold
-- 
------------------------------------------------------------------
Reinhold Kainhofer, address@hidden, http://reinhold.kainhofer.com/
 * Financial & Actuarial Math., Vienna Univ. of Technology, Austria
 * http://www.fam.tuwien.ac.at/, DVR: 0005886
 * LilyPond, Music typesetting, http://www.lilypond.org

Issue_RemovingNonExistingListener.ly
Description: Text Data