proposition: Lemma big_mset0 P F : \big[op/idx]_(i <- mset0 | P i) F i = idx.
proof: Proof. by rewrite enum_mset0 big_nil. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap