\version "2.15.9"

#(define-public (test_engraver context)
   (let ((string-pitch-list '()) ;; actually not used
         (notes '()))
   `((acknowledgers
       (note-head-interface .
         ,(lambda (trans grob source)
            (set! notes (cons grob notes)))))
     (process-acknowledged
       . ,(lambda (trans)
            (for-each (lambda (note)
              (let* ((event (event-cause note))
                     (pitch (ly:event-property event 'pitch))
                     (art (ly:event-property event 'articulations))
		     ;; art is a LIST of events, so you have to filter out 
		     ;; all string-number events
		     (nrs (filter (lambda (s) (not (null? s)))
				  (map (lambda (a) (ly:event-property a 'string-number)) 
				       art)))
                     ;;(type (ly:music-property art 'string-number)) <- this does not work
                     )
		    (display (extract-named-music art "ArticulationEvent"))
                    (display "The event: ")
		    ;;(display event)
                     (display " has pitch: ")
		     ;;(display pitch)
                     (display " and articulation: ")
                     (display art)
                     (display " and string numbers: ")
		     (display nrs)
                    (display " \n")
              )
              ) ;; end lambda (note)
              notes)))
     (stop-translation-timestep
       . ,(lambda (trans) (set! notes '()))))))

\layout {
   \context {
     \Voice
     \consists \test_engraver
   }
}

test = \relative c' {
%   c4 
  <d\2-.> %e\2 
%   f g\4 a\4 b\4 < g\4\harmonic >
%   c4\3 < d\3 g\2> c\3 r
}
%\displayMusic { \test }
\new Voice { \test }