proposition: Lemma addrK_char2 x : involutive (+%R^~ x).
proof: Proof. by move=> y; rewrite /= -subr_char2 addrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)