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CoGroupByKey
------------

CoGroupByKey performs a relational join of two or more key/value PCollections that have the same 
key type.

**Kata:** Implement a 
[CoGroupByKey](https://beam.apache.org/releases/pydoc/current/apache_beam.transforms.util.html#apache_beam.transforms.util.CoGroupByKey) 
transform that join words by its first alphabetical letter, and then produces the string 
representation of the WordsAlphabet model.

<div class="hint">
  Refer to
  <a href="https://beam.apache.org/releases/pydoc/current/apache_beam.transforms.util.html#apache_beam.transforms.util.CoGroupByKey">
    CoGroupByKey</a> to solve this problem.
</div>

<div class="hint">
  Refer to the Beam Programming Guide
  <a href="https://beam.apache.org/documentation/programming-guide/#cogroupbykey">
    "CoGroupByKey"</a> section for more information.
</div>
