have : under (lh ∅ : M) = under 0 := by simpa using Eq.symm <| Seq.domain_eq (M := M) (s := ∅) (by simp)
M : Type u_1
inst✝⁵ : Zero M
inst✝⁴ : One M
inst✝³ : Add M
inst✝² : Mul M
inst✝¹ : LT M
inst✝ : M ⊧ₘ* 𝐈𝚺₁
⊢ lh ∅ = 0
M : Type u_1
inst✝⁵ : Zero M
inst✝⁴ : One M
inst✝³ : Add M
inst✝² : Mul M
inst✝¹ : LT M
inst✝ : M ⊧ₘ* 𝐈𝚺₁
this : under (lh ∅) = under 0
⊢ lh ∅ = 0