$$\newcommand{\sslash}{\mathbin{/\mkern-6mu/}}$$ $$\newcommand{\defeq}{\stackrel{\text{def}}{=}}$$ $$\require{bussproofs}$$ $$\newcommand{\counterfactual} {~\Box\kern-1.5pt\raise1pt\hbox{$\mathord{\rightarrow}$}~} \newcommand{\countermight} {~\mathord{\Diamond}\kern-1.5pt\raise0.5pt\hbox{$\mathord{\rightarrow}$}~}$$