simp only [ncols, Schema.length_eq_List_length, Schema.nths,
           Schema.map_eq_List_map]
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
ns : List (Fin (ncols t))
⊢ ncols (selectColumns2 t ns) = List.length ns
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
ns : List (Fin (ncols t))
⊢ List.length (List.map (fun n => Schema.nth sch n.val (_ : n.val < Schema.length sch)) ns) = List.length ns