Thanks for the answers. Maybe I should have explained why I needed that. What I had in mind, is to be able to detect subgraphs. But not any kind of subgraphs, but only those on which ALL objects are ONLY reachable from the root of the subgraph. This means, that there are no objects outside the graph, pointing to objects inside the graph. In ImageSegment word, would mean to detect subgraphs that don't have outPointers.
So why not simply use an ImageSegment? IIRC that's how Croquet islands are checked for closedness.