proposition: Lemma big_mkcond_idem I r (P : pred I) F : \big[op/x]_(i <- r | P i) F i = \big[op/x]_(i <- r) (if P i then F i else x).
proof: Proof. elim: r => [|i r]; rewrite ?(big_nil, big_cons)//. by case: ifPn => Pi ->//; rewrite -[in LHS]big_id_idem // opC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)