proposition: Lemma prelift_opp s : prelift (prefreeg_opp s) = - prelift s.
proof: Proof. by rewrite [LHS]big_map -sumrN; apply: eq_bigr => i _; rewrite scaleNr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)