next _ target heq =>
  have inv' : LowerInvExploreεClosure nfa next target stack.pop := by
    intro i j hi hj
    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)
  exact lower_inv_exploreεClosure h inv'
case h_1
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')
⊢ LowerInvεClosure nfa next' #[]

case h_2
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
save✝ : Array (Option String.Pos)
heq✝ : stack.back' hemp = StackEntry.restore save✝
h : εClosure nfa pos next save✝ matched saveSlots stack.pop = (matched', next', saveSlots')
⊢ LowerInvεClosure nfa next' #[]
case h_2
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
save✝ : Array (Option String.Pos)
heq✝ : stack.back' hemp = StackEntry.restore save✝
h : εClosure nfa pos next save✝ matched saveSlots stack.pop = (matched', next', saveSlots')
⊢ LowerInvεClosure nfa next' #[]