\{!$x$:$T$ $\mid$ $P$($x$)\} $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\{$x$:$T$$\mid$ $P$($x$) \& ($\forall$$y$:$T$. $P$($y$) $\Rightarrow$ $y$ $=$ $x$) \}