$\forall$$T$:Type, $P$:($T$$\rightarrow\mathbb{P}$), $d$:(\{$i$:$T$$\mid$ $P$($i$)\}  List), $x$:\{$i$:$T$$\mid$ $P$($i$)\} . ($x$ $\in$ $d$) $\Rightarrow$ ($x$ $\in$ $d$)