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Flatten
-------

Flatten is a Beam transform for PCollection objects that store the same data type. Flatten merges
multiple PCollection objects into a single logical PCollection.

**Kata:** Implement a
[Flatten](https://godoc.org/github.com/apache/beam/sdks/go/pkg/beam#Flatten)
transform that merges two PCollection of words into a single PCollection.

<div class="hint">
  Refer to <a href="https://godoc.org/github.com/apache/beam/sdks/go/pkg/beam#Flatten">
  Flatten</a> to solve this problem.
</div>

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