func (c *greeterServiceProtobufClient) callSign(ctx context.Context, in *SignRequest) (*SignResponse, error) {
    out := new(SignResponse)
    ctx, err := doProtobufRequest(ctx, c.client, c.opts.Hooks, c.urls[1], in, out)
    if err != nil {
        twerr, ok := err.(twirp.Error)