code: func WithRCSBackendString(ctx context.Context, sb string) context.Context {
  	if be := rcsBackendFromName(sb); be >= 0 {
  		return WithRCSBackend(ctx, be)
  	}
  	return WithRCSBackend(ctx, Noop)
  }
docs: // WithRCSBackendString returns a context with the given sync backend set