In POPs there is the idea of an active stream. This is the set of particles a micro solver should work on by virtue of its inputs. The
opstreamname expression computes the ad hoc point group for the nodes active stream.
If the node has only a single solid input that leads to a generator, this will evaluate to the
streamname parameter of that generator. But, in the presence of merge nodes, it may produce a list of groups that form the active stream.