es{-}first{-}since(${\it es}$;$e$.$P$($e$);$e_{1}$;$e_{2}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$es{-}le(${\it es}$; $e_{1}$; $e_{2}$) $\wedge$ $P$($e_{2}$) $\wedge$ all$e$ from ${\it es}$ in [$e_{1}$;$e_{2}$).$\neg$$P$($e$)