Are stateful transducers atop `org.apache.kafka.streams.state` implemented, or planned?

[blak3mill3r]

In the README, it says

This library also provides a number of stateful transducers over Kafka Streams' Stores API for doing joins,
windowed aggregates, etc. The goal of this library is to maintain feature parity with the high-level KStream,
KTable, etc. APIs, as well as (eventually) to enable transducer usage in the low-level Processor API.

If this implemented in master right now, it was not obvious to me.

Although I see some things that look like building-blocks for the above idea (KeyValueTupleIterator, IReadOnlyKeyValueStore), I did not see any definitions of stateful transducers using those primitives, and no test or example code with kafka-backed stateful transducers.

I think this has the potential to be very powerful. If it is not implemented yet, I think I'll have a crack at it, but I wanted to ask for clarification first. I'd be happy to chat about it, too.