This turned out to be extremely useful in many map-reduce scenarios where we do lengthy computations that output data that is used as a complete and consistent set. For example, calculating average prices across ranges of items (the example in the documentation). Another good use can be for tags stored as arrays of strings – you want to know how many items are tagged with a given entry. We can now take my incremental tag count map-reduce and replace it with a much more trivial implementation that uses this gem.
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