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