es{-}stable(${\it es}$;$i$;${\it state}$.$P$(${\it state}$))
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$alle{-}at(${\it es}$;$i$;$e$.$P$(es{-}state{-}when(${\it es}$;$e$)) $\Rightarrow$ $P$(es{-}state{-}after(${\it es}$;$e$)))