cases inv i j hi hj with
| inl hnext => exact .inl hnext
| inr hstack =>
  cases Array.mem_pop_or_eq_of_mem _ _ hemp hstack with
  | inl hstack => exact .inr (.inr hstack)
  | inr heq' =>
    rw [heq] at heq'
    have : j = target := by
      simp at heq'
      exact heq'
    exact .inr (.inl this)
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
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 : LowerInvεClosure nfa next stack
hemp : ¬stack.isEmpty = true
this : stack.pop.size < stack.size
entry✝ : StackEntry nfa.nodes.size
target : Fin nfa.nodes.size
heq : stack.back' hemp = StackEntry.explore target
h : exploreεClosure nfa pos next currentSave matched saveSlots target stack.pop = (matched', next', saveSlots')
i j : Fin nfa.nodes.size
hi : i ∈ next
hj : ↑j ∈ nfa.εStep ↑i
⊢ j ∈ next ∨ j = target ∨ StackEntry.explore j ∈ stack.pop
no goals