$\forall$$T$, ${\it T'}$:Type, $L$:$T$ List, $y$:${\it T'}$, $f$:(\{$x$:$T$$\mid$ ($x$ $\in$ $L$) \}$\rightarrow$${\it T'}$).
\\[0ex]($y$ $\in$ mapl($f$;$L$)) $\Leftrightarrow$ ($\exists$$a$:$T$. ($a$ $\in$ $L$) \& $y$ $=$ $f$($a$))