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Combine - BinaryCombineFn Lambda
--------------------------------

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

Since Beam v2.13.0, you can also use lambda or method reference in order to create the
BinaryCombineFn.

**Kata:** Implement the summation of BigInteger using lambda or method reference.

<div class="hint">
  Refer to
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/SerializableBiFunction.html">
    SerializableBiFunction</a>.
</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>
