The initial firings of A and B jointly cause F to fire. But there is dependence on neither A nor B: If A hadn’t fired, B would have caused both D and E to fire, and each in turn would have sent one of the two needed signals to F. Similarly if B hadn’t fired.

Here, as in your example, we can save the account if we allow that a “reduction” of the actual situation can remove neurons themselves — thinking of the presence of these neurons as being a ‘deviation’ from a default state in which nothing is there at all. (That strikes me as unattractive, for various reasons I’ll skip over.) But otherwise there’s trouble: no nomologically possible situation in which strictly fewer firings happen is one in which F depends on A, or on B.

I’d thought it would be easy to use flowchart type things, but I just found it easier to draw regular circles and arrows.

15 years ago I could have done this thing in my sleep using Corel Draw. But I don’t even know if that still exists, and if it does it has probably got so fancy that it is hard to do simple diagrams with. In any case, I don’t have a copy of any version. (An old version of Corel Draw 3 or 5 might come in handy sometimes, so maybe I should try to dig one up!)