I've been writing a simple 2d arcade rpg a few years ago. I created a simple graph language for creating quests and started to design the system for specifying arbitrary quest conditions.

It started from simple things like "has player killed a monster of type X", "does player have item Y in inventory", "was the actor X in place Y already", "have player talked to actor Z about Q".

But then I thought it would be cool to add metainformation. "Have actor X seen player killing the monster Y". "Have any actor seen player talking about Q". "is there a chain of actors meeting each other from the actor that seen player killing actor X to the quest giver".

I tried to implement this naively and it turns out the amount of information is astounding. To the point that I'm still amazed our brains can do this.