[Inform 7] Determining if a relation is defined for a value.

Assuming I have defined a relation between two values but have not yet specified the relation for a specific value, is there a condition I can test that will tell me the relation hasn’t been defined yet?

For example:

Following relates various scenes to one scene (called the sequel). The verb to follow means the reversed following relation.

Scene A is a scene.  Scene B is a scene.  Scene B follows Scene A.

This means Scene B has no sequel but I can’t figure out how to test for that. Any ideas?



“if no scene follows Scene B” should work.

ETA: also “If no scene is the sequel of scene B.” There might be other ways too!

Here’s how I’d do it.

Definition: a scene is preceding rather than final if a scene follows it.

Now a scene is “final” if nothing follows it.