tion", "secret")
            .build();
      }
    }

    return null; // Didn't find a preemptive auth scheme.
 </code></pre>

 <h3>Reactive Authentication</h3>

 <p>Implementations authenticate by returning a follow-up request that includes an authorization
 header, or they may decline the challenge by returning null. In this case the unauthenticated
 response will be returned to the caller that triggered it.

 <p>Implementations should check if the initial request already included an attempt to
 authenti