contradiction
α nt : Type
P : Finset α → Finset nt → Type
i : Production.ContextFree α nt P
G : Grammar P
lhs xs : Word ({ x // x ∈ G.V } ⊕ { x // x ∈ G.Z })
a : { x // x ∈ G.Z }
step : Grammar.DerivationStep G lhs
h_lhs : lhs = Sum.inr a :: xs
hpre : step.pre = []
sound : Sum.inr a = Sum.inl (Production.ContextFree.lhs_var ↑step.prod) ∧ xs = step.suf
left✝ : Sum.inr a = Sum.inl (Production.ContextFree.lhs_var ↑step.prod)
right✝ : xs = step.suf
⊢ [] = Sum.inr a :: List.tail []
no goals