code: def _getInstrumentsVoc(self):
          
          cfilter = {'portal_type': 'Instrument', 'is_active': True}
          if self.getMethod():
              cfilter['getMethodUIDs'] = {"query": self.getMethod().UID(),
                                          "operator": "or"}
          bsc = getToolByName(self, 'bika_setup_catalog')
          items = [('', 'No instrument')] + [
              (o.UID, o.Title) for o in
              bsc(cfilter)]
          o = self.getInstrument()
          if o and o.UID() not in [i[0] for i in items]:
              items.append((o.UID(), o.Title()))
          items.sort(lambda x, y: cmp(x[1], y[1]))
          return DisplayList(list(items))
docs: This function returns the registered instruments in the system as a
          vocabulary. The instruments are filtered by the selected method.