$\vdash$  $\forall$$b$, $F$:Top, $n$, $m$:$\mathbb{N}$. primrec($n$+$m$;$b$;$\lambda$$i$.$F$) $\sim$ primrec($n$;primrec($m$;$b$;$\lambda$$i$.$F$);$\lambda$$i$.$F$)