Hi squirrels, when porting the gelly streaming code from 0.9 to 0.10 today with Paris, we hit an exception in union: "*A DataStream cannot be unioned with itself*".
The code raising this exception looks like this: stream.union(stream.map(...)). Taking a look into the union code, we see that it's now not allowed to union a stream, not only with itself, but with any product of itself. First, we are wondering, why is that? Does it make building the stream graph easier in some way? Second, we might want to give a better error message there, e.g. "*A DataStream cannot be unioned with itself or a product of itself*", and finally, we should update the docs, which currently state that union a stream with itself is allowed and that "*If you union a data stream with itself you will still only get each element once.*" Cheers, -Vasia.
