Section big.
Variables (R : Type) (idx : R) (op : Monoid.law idx).
Implicit Types (X : {mset K}) (P : pred K) (F : K -> R).
Hint Resolve msubset_refl : core.
Arguments msubset_trans {C A B} _ _ : rename.
Definition size_mset A : size A = \sum_(a <- finsupp A) A a.

Lemma size_mset_eq0 A : (size A == 0) = (A == mset0).