$\forall$$A$, $B$, $C$:Type, $f$:($A$$\rightarrow$$B$), $g$:($B$$\rightarrow$$C$), ${\it as}$:$A$ List. map($g$;map($f$;${\it as}$)) $=$ map($g$$\,\circ\,$$f$;${\it as}$) $\in$ $C$ List