proposition: Fact addfxA : associative subfext_add.
proof: Proof. by move=> x y t; rewrite -[x]reprK -[y]reprK -[t]reprK !piE addrA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype tuple finfun bigop ssralg countalg) mathcomp(finalg zmodp matrix vector falgebra poly polydiv) mathcomp(mxpoly generic_quotient)