proposition: Lemma big_set0 F : \big[op/x]_(i in set0) F i = x.
proof: Proof. by apply: big_pred0 => i; rewrite inE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)