rw [mul_cons_trivial] at H
case refl.refl
α : Type u
inst✝¹ : DecidableEq α
inst✝ : LT α
a a0 : α
M X : Multiset α
H✝ : a ::ₘ M = X + {a0}
hyp : ¬a = a0
a0_a : a0 ≠ a
H : Multiset.count a0 (a ::ₘ M) = Multiset.count a0 (X + {a0})
ba : ¬a0 = a
⊢ Multiset.count a0 (a ::ₘ M) = Multiset.count a0 X + 1
case refl.refl
α : Type u
inst✝¹ : DecidableEq α
inst✝ : LT α
a a0 : α
M X : Multiset α
H✝ : a ::ₘ M = X + {a0}
hyp : ¬a = a0
a0_a : a0 ≠ a
H : Multiset.count a0 (a ::ₘ M) = Multiset.count a0 (a0 ::ₘ X)
ba : ¬a0 = a
⊢ Multiset.count a0 (a ::ₘ M) = Multiset.count a0 X + 1