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Combine - BinaryCombineFn
-------------------------

Combine is a Beam transform for combining collections of elements or values in your data. When you 
apply a Combine transform, you must provide the function that contains the logic for combining the 
elements or values. The combining function should be commutative and associative, as the function 
is not necessarily invoked exactly once on all values with a given key. Because the input data 
(including the value collection) may be distributed across multiple workers, the combining function 
might be called multiple times to perform partial combining on subsets of the value collection.

BinaryCombineFn is used for implementing combiners that are more easily expressed as binary 
operations.

**Kata:** Implement the summation of BigInteger using 
[Combine.BinaryCombineFn](https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/Combine.BinaryCombineFn.html).

<div class="hint">
  Extend the
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/Combine.BinaryCombineFn.html">
    Combine.BinaryCombineFn</a> class that counts the sum of the number.
</div>

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