ck finsupp_msetn /= enum_fsetE /= enum_fset1 /= cats0.
by rewrite msetE eqxx.

(* Specifications: *)
Lemma msetKDI A B : A `&` (A `+` B) = A.
(* adjunction / subtraction *)
Qed.
Proof. by rewrite mproperEneq; case
Lemma mset1P x a : reflect (x = a) (x \in [mset a]).
Qed.