twitter/scalding — pull request #171 by johnynek (2012-09-01 23:55:55+00:00)
Title: Further refactors GroupBuilder methods

There are now three traits; ReduceOperations, FoldOperations, StreamOperations.
This is clearing the path for doing groupBy within a groupBy and for
better compile-time safety if we introduce aggregateBy with an AggregateBuilder
that only knows about ReduceOperations.