scheme@(guile-user)> (pretty-print (filter (lambda (p) (assoc-ref p 'instrument?)) (plugins)))
(((id . "http://drobilla.net/plugins/mda/DX10")
  (name . "MDA DX10")
  (instrument? . #t))
 ((id . "http://drobilla.net/plugins/mda/EPiano")
  (name . "MDA ePiano")
  (instrument? . #t))
 ((id . "http://drobilla.net/plugins/mda/JX10")
  (name . "MDA JX10")
  (instrument? . #t))
 ((id . "http://drobilla.net/plugins/mda/Piano")
  (name . "MDA Piano")
  (instrument? . #t)))
scheme@(guile-user)>