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