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<h2>Flatten</h2>
<p>Flatten is a Beam transform for PCollection objects that store the same data type.
  Flatten merges multiple PCollection objects into a single logical PCollection.</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/Flatten.html">
    Flatten</a> transform that merges two PCollection of words into a single PCollection.
</p>
<br>
<br>
<div class='hint'>Refer to
  <a href="https://beam.apache.org/releases/javadoc/2.11.0/org/apache/beam/sdk/transforms/Flatten.html">Flatten</a>
  to solve this problem.</div>
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