Hi Mengqi,
if what you are trying to do is output the solution set of every iteration, before the iteration has finished, then that is not possible.
i.e. you can not output the solution set to a sink or another operator during the iteration.
However, you can add elements to the solution set and grow the dataset as the iteration proceeds and retrieve it after it is finished.
Currently, you cannot union with the solution set, but a CoGroup could give you the desired result.
Alternatively, you might want to look at bulk iterations (Iterate abstraction) [1]. With these you can produce a completely new dataset after every iteration.
I hope that helps!
-Vasia.
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