induction' t using Quotient.inductionOn with l₂
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
case h.h
α : Type u_1
s t : Multiset α
n : ℕ
l₀ : List α
hs : card ⟦l₀⟧ ≤ n
l₂ : List α
ht : n ≤ card ⟦l₂⟧
hst : ⟦l₀⟧ ≤ ⟦l₂⟧
⊢ ∃ u, ⟦l₀⟧ ≤ u ∧ u ≤ ⟦l₂⟧ ∧ card u = n