Reacting to visibility
Two facts let a story read what is currently visible, so an interaction can behave differently depending on who can see whom.
This counts how many other players this player currently sees with an effect on them. Use it to check whether someone's detection power is finding anything, or to trigger dialogue the first time a player can see a hidden spirit.
The second one counts the other direction: how many players currently see this player with an effect. Use it to check whether a vanished player really is hidden from the whole server, or to count how many players a disguise is fooling.
Both facts have a Rule Entry field. Leave it empty and the fact counts every rule that is currently active. Point it at one visibility rule entry and it counts only the pairs that rule created, which is what you want when several rules are running at once and you only care about one of them.
An effect a player has on their own view of themselves is never counted, so the viewers count still reaches zero while a player sees themselves changed.