es{-}x{-}equiv(${\it es}$; $i$; $x$; $s_{1}$; $s_{2}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$z$:Id. ($\neg$($z$ = $x$)) $\Rightarrow$ ($s_{1}$($z$) = $s_{2}$($z$))