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)
case inr
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
hstack : StackEntry.explore j ∈ stack
⊢ j ∈ next ∨ j = target ∨ StackEntry.explore j ∈ stack.pop
no goals