$\forall$$T$, ${\it T'}$:Type, $a$:($T$ List), $f$, $g$:($T$$\rightarrow$${\it T'}$).
\\[0ex]($\forall$$x$:$T$. ($x$ $\in$ $a$) $\Rightarrow$ ($f$($x$) = $g$($x$))) $\Rightarrow$ (map($f$;$a$) = map($g$;$a$) $\in$ (${\it T'}$ List))