compound
\\[0ex]rng\_hom\_p($r$;$s$;$f$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$FunThru2op($\mid$$r$$\mid$;$\mid$$s$$\mid$;+$r$;+$s$;$f$) \& FunThru2op($\mid$$r$$\mid$;$\mid$$s$$\mid$;$\ast$$r$;$\ast$$s$;$f$) \& $f$(1$r$) = (1$s$) $\in$ $\mid$$s$$\mid$