ma{-}x{-}equiv($M$;$x$;$s_{1}$;$s_{2}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$z$:Id. ($\neg$($z$ = $x$ $\in$ Id)) $\Rightarrow$ ($s_{1}$($z$) = $s_{2}$($z$) $\in$ $M$.ds($z$))