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