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.

Lemma msetDBA A B C : C `<=` B -> A `+` B `\` C = (A `+` B) `\` C.