proposition: Lemma eq_fbig (A B : {fset I}) (F G : I -> R) : A =i B -> (forall x, x \in A -> F x = G x) -> \big[op/idx]_(i <- A) F i = \big[op/idx]_(i <- B) G i.
proof: Proof. by move=> AB FG; apply: eq_fbig_cond => x; rewrite ?inE/= -?AB// => /FG. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)