proposition: Lemma big_seq_fset0 (F : I -> R): \big[op/idx]_(i <- fset0) F i = idx.
proof: Proof. by rewrite big_seq_fsetE big_fset0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)