left
α : Type
R : RegExp α
cs : List α
a2_left : ∀ r ∈ [], r ∈ RegExp.languageOf α R
a2_right : [].join = cs
⊢ [].join = [] ∨ ∃ r ∈ RegExp.languageOf α R, ∃ a, (∀ r ∈ a, r ∈ RegExp.languageOf α R) ∧ r ++ a.join = [].join
case h
α : Type
R : RegExp α
cs : List α
a2_left : ∀ r ∈ [], r ∈ RegExp.languageOf α R
a2_right : [].join = cs
⊢ [].join = []