$\forall$$T$:Type, $L$:$T$ List, $f$, $g$:($\mathbb{N}$$_{\mbox{\scriptsize $<\parallel$$L$$\parallel$}}$$\rightarrow\mathbb{N}$$_{\mbox{\scriptsize $<\parallel$$L$$\parallel$}}$). (($L$ o $f$) o $g$) $=$ ($L$ o $f$$\,\circ\,$$g$) $\in$ $T$ List