rw [List.length_map, List.zip_length_eq_of_length_eq]
η : Type u_η
dec_η : DecidableEq η
sch : Schema
τ : Type u
t : Table sch
c : η
f : Row sch → Option τ
⊢ List.length (List.map (fun x => Row.addColumn x.fst c x.snd) (List.zip t.rows (List.map f t.rows))) =
    List.length t.rows
case h
η : Type u_η
dec_η : DecidableEq η
sch : Schema
τ : Type u
t : Table sch
c : η
f : Row sch → Option τ
⊢ List.length t.rows = List.length (List.map f t.rows)