rw [nths, Schema.map_eq_List_map]
η : Type u_η
dec_η : DecidableEq η
schema : Schema
α : Type u_1
xs : List α
ns : List (Fin (length xs))
⊢ List.length (nths xs ns) = List.length ns
η : Type u_η
dec_η : DecidableEq η
schema : Schema
α : Type u_1
xs : List α
ns : List (Fin (length xs))
⊢ List.length (List.map (fun n => nth xs n.val (_ : n.val < length xs)) ns) = List.length ns