$\vdash$  $\forall$$n$, $m$:$\mathbb{N}$, $f$, $x$:Top. ($f$$^{\mbox{\scriptsize $n$+$m$}}$($x$)) $\sim$ ($f$$^{\mbox{\scriptsize $n$}}$($f$$^{\mbox{\scriptsize $m$}}$($x$)))