Hi Jonathan, if I follow your formalism correctly, Pb(b(x)) is a complex event which realizes Fp in the relevant situation, and Pb(b(x))O is a part of the former (it's "the last event in Pb(b(x))"). But Pb(b(x)) cannot be "what leads to Pb(b(x))O" as you claim at the end if the latter is part of the former. When you fill a glass of water to the top, the whole filling-to-the-top event doesn't determine, cause or lead to the adding-the-last-drop event. I suspect the problem here is that you're thinking of the last event both as "the last event in [the chain]" and what "follow[s] the chain".