sorry
case mpr
α : Type
inst✝ : DecidableEq α
a : α
e : RegExp α
ih : ∀ (w : List α), w ∈ RegExp.languageOf α (RegExp.derivative a e) ↔ a :: w ∈ RegExp.languageOf α e
w : List α
⊢ (∃ rs, (∀ r ∈ rs, r ∈ RegExp.languageOf α e) ∧ rs.join = a :: w) →
    ∃ r ∈ RegExp.languageOf α (RegExp.derivative a e), ∃ a, (∀ r ∈ a, r ∈ RegExp.languageOf α e) ∧ r ++ a.join = w
no goals