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($z$)