).
Proof. by rewrite msetE2 msetnE; case: (b == a). Qed.

Lemma msetB1E a A b : (A `\ a) b = (A b) - (b == a).
Proof. by rewrite msetE2 msetnE;.
Qed.

Lemma in_msetM A B (u : K * K) : (u \in A `*` B) = (u.1 \in A) && (u.2 \inB).
Proof. by rewrite -!msuppE !mem_finsupp msetE muln_eq0 negb_or. Qed.