code: func OpenRCS(ctx context.Context, id RCSBackend, path string) (RCS, error) {
  	if be, found := rcsRegistry[id]; found {
  		return be.Open(ctx, path)
  	}
  	return nil, errors.Wrapf(ErrNotFound, "unknown backend: %d", id)
  }
docs: // OpenRCS opens an existing repository.