have H0 : Multiset.count x M + Multiset.count x N = Multiset.count x P + Multiset.count x Q := by
  rw [Multiset.ext] at h
  simp_all only [Multiset.mem_add, Multiset.count_add]
case a
α : Type u_1
inst✝ : Preorder α
dec : DecidableEq α
M N P Q : Multiset α
h : M + N = P + Q
x : α
⊢ Multiset.count x N = min (Multiset.count x N) (Multiset.count x Q) + (Multiset.count x P - Multiset.count x M)
case a
α : Type u_1
inst✝ : Preorder α
dec : DecidableEq α
M N P Q : Multiset α
h : M + N = P + Q
x : α
H0 : Multiset.count x M + Multiset.count x N = Multiset.count x P + Multiset.count x Q
⊢ Multiset.count x N = min (Multiset.count x N) (Multiset.count x Q) + (Multiset.count x P - Multiset.count x M)