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 subset_msetBLR A B C : (msubset (A `\` B) C) = (A `<=` B `+` C).