compound
\\[0ex]rng\_chom\_p($r$;$s$;$f$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$rng\_hom\_p($r$;$s$;$f$) \& ($\forall$$a$, $b$:$\mid$$r$$\mid$. (($f$($a$)) $\ast$ ($f$($b$))) = (($f$($b$)) $\ast$ ($f$($a$))))