Earlier quoted context omitted.
You probably need to come up with a statement that’s true, but is made false by the act of getting B.
So, "I don't have B"? But the system is mutable, and we can easily see that the proposition is true, and then (possibly) false. Then you have to say "I will never have B". But that's not necessarily true! You would need either an oracle, some sort of iterative evaluation, or a perspective from which the entire interaction is a single static object, something like a tree of outcomes, which you allow to be used in the…
Good enough. All this sort of thing becomes a Genie problem if you think about it too much. We all get what the solution means.