## CoGroupByKey

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

Kata: Implement a CoGroupByKey transform that join words by its first alphabetical letter, and then produces the toString() representation of the WordsAlphabet model.

Refer to CoGroupByKey, TupleTag, and CoGbkResult.
Refer to the Beam Programming Guide "CoGroupByKey" section for more information.