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<h2>CoGroupByKey</h2>
<p>CoGroupByKey performs a relational join of two or more key/value PCollections that have the same key type.</p>
<p>In this task, we are going to implement a
    <a href="https://beam.apache.org/releases/javadoc/2.11.0/org/apache/beam/sdk/transforms/join/CoGroupByKey.html">
        CoGroupByKey</a> transform that join words by its first alphabetical letter, and then produces the toString()
    representation of the WordsAlphabet model.
</p>
<br>
<br>
<div class='hint'>Refer to <a href="https://beam.apache.org/releases/javadoc/2.11.0/org/apache/beam/sdk/transforms/join/CoGroupByKey.html">CoGroupByKey</a>,
    <a href="https://beam.apache.org/releases/javadoc/2.11.0/org/apache/beam/sdk/values/TupleTag.html">TupleTag</a>,
    and <a href="https://beam.apache.org/releases/javadoc/2.11.0/org/apache/beam/sdk/transforms/join/CoGbkResult.html">CoGbkResult</a>.</div>
</html>