code: func (k cursorKind) storePrevCursor(c context.Context, pkg, cursor, prev string) {
  	itm := memcache.NewItem(c, k.cursorKey(pkg, cursor))
  	itm.SetValue([]byte(prev))
  	itm.SetExpiration(24 * time.Hour)
  	if err := memcache.Set(c, itm); err != nil {
  		logging.WithError(err).Errorf(c, "Failed to store prev cursor %q in memcache", k)
  	}
  }
docs: // storePrevCursor stores mapping cursor => prev, so that prev can be fetched
  // later by fetchPrevCursor.
  //
  // Logs and ignores errors. Cursor mapping is non-essential functionality.