Airspace Adaptation, or the catalog of different items in the old ATC system, is nightmarishly complex. It allows for insane FPs that are just not suited for straight forward computation. Canada and Mexico also share fix names with US. From a math and physics perspective, the language of flight plans is degenerate and, I think, doesn't even hold group structure. It's the reason that humans are still required even though MIT's JEDI did a bang up job of conflict resolution, or keeping planes from giving each other hugs. on the nose. At 300 miles per hour.