$\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$^{+}$