apply Set.mem_of_mem_of_subset _ (assm i mem)
case mpr
nfa : NFA
S : Set ℕ
assm : ∀ i ∈ S, nfa.εClosure i ⊆ S
i : ℕ
mem : i ∈ S
x✝ : i < nfa.nodes.size
j : ℕ
step : j ∈ nfa[i].εStep
⊢ j ∈ S
nfa : NFA
S : Set ℕ
assm : ∀ i ∈ S, nfa.εClosure i ⊆ S
i : ℕ
mem : i ∈ S
x✝ : i < nfa.nodes.size
j : ℕ
step : j ∈ nfa[i].εStep
⊢ j ∈ nfa.εClosure i