cases path with
| base _ =>
  cases step with
  | charStep _ _ step => simp [NFA.Node.charStep, startNode] at step
  | εStep _ _ step =>
    simp [NFA.Node.εStep, startNode] at step
    cases step <;> simp only [neStart₁, neStart₂] at *
| @step _ _ _ cs₃ cs₄ step rest =>
  cases step with
  | charStep _ _ step => simp [NFA.Node.charStep, startNode] at step
  | εStep _ _ step =>
    simp at eqs
    simp [NFA.Node.εStep, startNode] at step
    cases step with
    | inl eqs₁ =>
      rw [eqs₁] at rest
      have : pathToNext final next nfa.nodes.size nfa₁.val.start.val cs :=
        ⟨i, cs₄, cs₂, eqs, eq₂ ▸ rest, step⟩
      have : pathToNext nfa₁ next nfa.nodes.size nfa₁.val.start.val cs := by
        apply this.cast' nfa₁.val.start.isLt
          (Nat.le_of_lt (Nat.lt_trans nfa₂.property final.property))
          get₁
      exact .alternateLeft (ih₁ eq₁.symm this)
    | inr eqs₂ =>
      rw [eqs₂] at rest
      have : pathToNext final next nfa.nodes.size nfa₂.val.start.val cs :=
        ⟨i, cs₄, cs₂, eqs, eq₄ ▸ rest, step⟩
      have : pathToNext nfa₂ next nfa.nodes.size nfa₂.val.start.val cs := by
        apply this.cast' nfa₂.val.start.isLt (Nat.le_of_lt final.property) get₂
      have : pathToNext nfa₂ next nfa₁.val.nodes.size nfa₂.val.start.val cs := by
        apply this.castLE (ge_pushRegex_start eq₃.symm)
        intro i j h₁ h₂ step hj
        have := eq_or_ge_of_step_pushRegex eq₃.symm h₁ h₂ step
        simp at this
        cases this with
        | inl eq =>
          exfalso
          rw [eq] at hj
          exact Nat.not_le_of_gt next.isLt hj
        | inr ge => exact ge
      exact .alternateRight (ih₂ eq₃.symm this)
case intro.intro.intro.intro.intro
nfa : NFA
next : Fin nfa.nodes.size
r₁ r₂ : Expr
result : { nfa' // nfa.nodes.size < nfa'.nodes.size }
cs : List Char
eq : nfa.pushRegex next (r₁.alternate r₂) = result
ih₁ :
  ∀ {nfa : NFA} {next : Fin nfa.nodes.size} {result : { nfa' // nfa.nodes.size < nfa'.nodes.size }} {cs : List Char},
    nfa.pushRegex next r₁ = result →
      (↑result).pathToNext (↑next) nfa.nodes.size (↑(↑result).start) cs → Expr.matches cs r₁
ih₂ :
  ∀ {nfa : NFA} {next : Fin nfa.nodes.size} {result : { nfa' // nfa.nodes.size < nfa'.nodes.size }} {cs : List Char},
    nfa.pushRegex next r₂ = result →
      (↑result).pathToNext (↑next) nfa.nodes.size (↑(↑result).start) cs → Expr.matches cs r₂
nfa₁ : { nfa' // nfa.nodes.size < nfa'.nodes.size }
start₁ : Fin (↑nfa₁).nodes.size
nfa₂ : { nfa' // (↑nfa₁).nodes.size < nfa'.nodes.size }
start₂ : Fin (↑nfa₂).nodes.size
inBounds : (Node.split ↑start₁ ↑start₂).inBounds ((↑nfa₂).nodes.size + 1)
final : { nfa' // (↑nfa₂).nodes.size < nfa'.nodes.size }
property : nfa.nodes.size < (↑final).nodes.size
eq₁ : nfa₁ = nfa.pushRegex next r₁
eq₂ : start₁ = (↑nfa₁).start
eq₃ : nfa₂ = (↑nfa₁).pushRegex ⟨↑next, ⋯⟩ r₂
eq₄ : start₂ = (↑nfa₂).start
eq₅ : final = (↑nfa₂).pushNode (Node.split ↑start₁ ↑start₂) inBounds
eq' : result = ⟨↑final, property⟩
startNode : (↑final)[↑(↑final).start] = Node.split ↑start₁ ↑start₂
neStart₁ : ↑next ≠ ↑start₁
neStart₂ : ↑next ≠ ↑start₂
get₂ : ∀ (i : ℕ), nfa.nodes.size ≤ i → ∀ (h₂ : i < (↑nfa₂).nodes.size), (↑nfa₂)[i] = (↑final)[i]
get₁ : ∀ (i : ℕ), nfa.nodes.size ≤ i → ∀ (h₂ : i < (↑nfa₁).nodes.size), (↑nfa₁)[i] = (↑final)[i]
i : ℕ
cs₁ cs₂ : List Char
eqs : cs = cs₁ ++ cs₂
path : (↑final).pathIn nfa.nodes.size (↑(↑final).start) i cs₁
step : (↑final).stepIn nfa.nodes.size i (↑next) cs₂
⊢ Expr.matches cs (r₁.alternate r₂)
no goals