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