proposition: Lemma addKr_char2 x : involutive (+%R x).
proof: Proof. by move=> y; rewrite -{1}[x]oppr_char2 addKr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)