docs: def serialize(self):
          """Get the unicode string representing the whole collection."""
          import datetime
          items = []
          time_begin = datetime.datetime.now()
          for href in self.list():
              items.append(self.get(href).item)
          time_end = datetime.datetime.now()
          self.logger.info(
              "Collection read %d items in %s sec from %s", len(items),
              (time_end - time_begin).total_seconds(), self.path)
          if self.get_meta("tag") == "VCALENDAR":
              collection = vobject.iCalendar()
              for item in items:
                  for content in ("vevent", "vtodo", "vjournal"):
                      if content in item.contents:
                          for item_part in getattr(item, "%s_list" % content):
                              collection.add(item_part)
                          break
              return collection.serialize()
          elif self.get_meta("tag") == "VADDRESSBOOK":
              return "".join([item.serialize() for item in items])
          return ""