have : stack.pop.size < stack.size := Array.lt_size_of_pop_of_not_empty _ (by assumption)
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
i : Fin nfa.nodes.size
inv : UpperInvεClosure nfa i stack
hemp : ¬stack.isEmpty = true
h :
  (match stack.back' hemp with
    | StackEntry.explore target => exploreεClosure nfa pos next currentSave matched saveSlots target stack.pop
    | StackEntry.restore save => εClosure nfa pos next save matched saveSlots stack.pop) =
    (matched', next', saveSlots')
⊢ UpperBoundεClosure nfa i next next'
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
i : Fin nfa.nodes.size
inv : UpperInvεClosure nfa i stack
hemp : ¬stack.isEmpty = true
h :
  (match stack.back' hemp with
    | StackEntry.explore target => exploreεClosure nfa pos next currentSave matched saveSlots target stack.pop
    | StackEntry.restore save => εClosure nfa pos next save matched saveSlots stack.pop) =
    (matched', next', saveSlots')
this : stack.pop.size < stack.size
⊢ UpperBoundεClosure nfa i next next'