case none   => exact absurd hp $ ne_of_ssubset e.progress_ssubset
α : Type
inst✝ : Proper α
s s₁ s₂ : State α
hp : s₁.progress = s₂.progress
tl₁ : State α
tail₂ : Tail s₂ s₂
n : State.Nontrivial s₁
ht : s₁.tag = s₂.tag
e : s₁ ↓ᵢ| s₂
⊢ s₁ = s₂
no goals