proposition: Lemma lregM x y : lreg x -> lreg y -> lreg (x * y).
proof: Proof. by move=> reg_x reg_y z t; rewrite -!mulrA => /reg_x/reg_y. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)