Variables (R : Type) (idx : R) (op : Monoid.com_law idx).
Implicit Types (X : {mset K}) (P : pred K) (F : K -> R).

(* Specifications: *)
Lemma big_mset X P F :
\big[op/idx]_(i <- X | P i) F i =
\big[op/idx]_(i <- finsupp X | P i) iterop (X i) op (F i) idx.