$\forall$$T$:Type, $f$:($T$$\rightarrow$$T$), $L$:($T$ List), $x$, $y$:$T$.
\\[0ex]$x$=$f$$\ast$($y$) via $L$ $\Rightarrow$ ($\forall$$a$:$T$. ($a$ $\in$ $L$) $\Rightarrow$ \{$x$ is $f$$\ast$($a$) \& $a$ is $f$$\ast$($y$)\})