Requested feature overview description.
"Set Implication" - Comparable to tag implication. Essentially, I'd like the option to say: "Any posts in X set should also be added to Y set". This is useful especially to me, since I have sets that are sub-categories of others and I'd like the higher category sets to contain the sub category set's posts.

Why would it be useful?
I believe this would be very useful to others like myself aswell, you are very into sets.
My Sets (For Example): https://e621.net/set?user_id=274335

What part(s) of the site page(s) are affected?
Sets mainly. Could kind of make the claim that it effects posts too.

I'd be willing to offer help with whatever would be needed to implement this.