proposition: Lemma big1_idem I r (P : pred I) F : (forall i, P i -> F i = x) -> \big[op/x]_(i <- r | P i) F i = x.
proof: Proof. move=> Fix; under eq_bigr => ? ? do rewrite Fix//; exact: big_const_idem. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)