proposition: Lemma some_big_AC_mk_monoid [I : Type] r P (F : I -> T) : Some (\big[op/x]_(i <- r | P i) F i) = oop (\big[oop/None]_(i <- r | P i) Some (F i)) (Some x).
proof: Proof. by elim/big_rec2 : _ => //= i [y|] _ Pi [] -> //=; rewrite opA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)