title: Applying a transform to one output tag
body: <p>I think I have a function which produces two outputs (please correct me if I'm wrong):</p>
  
  <pre><code>PCollection&lt;String&gt; words = ...;
  
  final TupleTag&lt;String&gt; shortWordsTag = new TupleTag&lt;String&gt;(){};
  
  PCollectionTuple results =
       words.apply(
           ParDo
           .of(new DoFn&lt;String, String&gt;() {
               @ProcessElement
               public void processElement(ProcessContext context) {
                   String word = context.element();
                   if (word.length() &lt; 5) {
                       context.output(shortWordsTag, word);
                   } else {
                       context.output(word);
               }
  </code></pre>
  
  <p>Now I'd like to call another function, but only apply it one of those outputs. Something like this:</p>
  
  <pre><code>results.apply(
      ParDo
      .of(new DoFn&lt;String, String&gt;() {
          @ProcessElement
          public void processElement(ProcessContext context) {
              String word = context.element();
              // do stuff, but should only have words with length &lt; 5 here
      }
  )
  </code></pre>
  
  <p>I can see some examples that use <code>withOutputTags</code> but this method seems to take more than one tag (a tag, and a list of tags), and I'm not sure how to use it for my scenario.</p>
  
  <p>How can I specify my <code>results.apply</code> to be only called for the data which is outputted to <code>shortWordsTag</code> tag?</p>