proposition: Lemma oppr_char2 x : - x = x.
proof: Proof. by apply/esym/eqP; rewrite -addr_eq0 addrr_char2. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)