es{-}causle(${\it es}$;$e$;${\it e'}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$es{-}causl(${\it es}$; $e$; ${\it e'}$) $\vee$ ($e$ = ${\it e'}$ $\in$ es{-}E(${\it es}$))