OptionalconstructCONSTRUCT query that produces the portal's subgraph.
Portal id. It is also the UGM edge id of the holarchy edge this portal projects to (issue #48), so it must be unique across every portal of the dataset; a repeated id keeps the first portal's edge and skips the later one, the way a portal to an unknown holon is already skipped. It should not reuse a holon id, because the canvas hands Cytoscape one id space for nodes and edges.
A portal connecting two holons.