cases SparseSet.eq_or_mem_of_mem_insert hi with
| inl htarget =>
  subst htarget
  simp [εStep, Node.εStep, hn] at hj
  exact .inr (.inl (Fin.eq_of_val_eq hj))
| inr hnext =>
  match inv i j hnext hj with
  | .inl hnext => exact .inl (SparseSet.mem_insert_of_mem hnext)
  | .inr (.inl htarget) => exact .inl (htarget ▸ SparseSet.mem_insert)
  | .inr (.inr hstack) => exact .inr (.inr hstack)
nfa : NFA
pos : String.Pos
next : SparseSet nfa.nodes.size
currentSave : Array (Option String.Pos)
matched : Option (Array (Option String.Pos))
saveSlots : Data.Vec (Array (Option String.Pos)) nfa.nodes.size
target : Fin nfa.nodes.size
stack : Array (StackEntry nfa.nodes.size)
matched' : Option (Array (Option String.Pos))
next' : SparseSet nfa.nodes.size
saveSlots' : Data.Vec (Array (Option String.Pos)) nfa.nodes.size
inv : LowerInvExploreεClosure nfa next target stack
h✝¹ : target ∉ next
offset✝ target' : ℕ
hn : nfa[↑target] = Node.save offset✝ target'
isLt : (Node.save offset✝ target').inBounds nfa.nodes.size
h✝ : ¬offset✝ < currentSave.size
h :
  exploreεClosure nfa pos (next.insert target) currentSave matched saveSlots ⟨target', ⋯⟩ stack =
    (matched', next', saveSlots')
i j : Fin nfa.nodes.size
hi : i ∈ next.insert target
hj : ↑j ∈ nfa.εStep ↑i
⊢ j ∈ next.insert target ∨ j = ⟨target', isLt⟩ ∨ StackEntry.explore j ∈ stack
no goals