$\forall$$A$, $B$, $C$, $D$:Type, $f$:($A$$\rightarrow$$B$), $g$:($B$$\rightarrow$$C$), $h$:($C$$\rightarrow$$D$). ($h$ o ($g$ o $f$)) = (($h$ o $g$) o $f$) $\in$ $A$$\rightarrow$$D$