fact: Definition pair_unitr := [qualify a x : R1 * R2 | (x.1 \is a GRing.unit) && (x.2 \is a GRing.unit)].
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)