I’d like to re-raise the issue of exclusive or (xor) relationships such that:
Each vehicle must be at one and only one facility XOR each vehicle must be on one and only one route. This is often shown as an arc on a logical data model diagram. In the transition to PER, this would be translated to two foreign key columns with a trigger that would enforce the presence of exactly one of the two FK columns. I read the workaround in an earlier post, but that workaround suggests that the two entities are subtypes of a third–which is redirecting the “reality” from the relationships (where it belongs) to the entities.