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}$) \& $P$($e_{2}$) \& alle{-}between1(${\it es}$;$e_{1}$;$e_{2}$;$e$.$\neg$$P$($e$))