induction' s using Quotient.inductionOn with l₀
α : Type u_1
s t : Multiset α
n : ℕ
hst : s ≤ t
hs : card s ≤ n
ht : n ≤ card t
⊢ ∃ u, s ≤ u ∧ u ≤ t ∧ card u = n
case h
α : Type u_1
s t : Multiset α
n : ℕ
ht : n ≤ card t
l₀ : List α
hst : ⟦l₀⟧ ≤ t
hs : card ⟦l₀⟧ ≤ n
⊢ ∃ u, ⟦l₀⟧ ≤ u ∧ u ≤ t ∧ card u = n