Comment 1 for bug 607040

NodeContextTracker is meant to be a class to track node contexts (incidence information) in a generic graph model. Creating more that one arc between nodes is not an invalid operation in general case.

Therefore, the correct behaviour is to remove nodes from pre-/post-sets when all connections involving the two nodes are removed.

Models that need more involved context tracking should not use the default context tracker.

Forbidding multiple arcs between the same node should only be implemented in Petri nets/STGs.