EqDecider($T$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$${\it eq}$:($T$$\rightarrow$$T$$\rightarrow\mathbb{B}$)$\times$($\forall$$x$, $y$:$T$. $x$ $=$ $y$ $\Leftrightarrow$ ${\it eq}$($x$,$y$))