Implement extract once

exercises/practice/matrix/.meta/example.sml

@@ -4,20 +4,13 @@ local
 
   val cells: string -> string list =
     String.tokens (fn (c: char) => c = #" ")
+
+    fun extract (index: int) (l: string list): string =
+      List.nth (l, index - 1)
 in
   fun row (s: string, index: int): int list =
-    let
-      fun extract (l: string list): string =
-        List.nth (l, index - 1)
-    in
-      map (valOf o Int.fromString) ((cells o extract o lines) s)
-    end
+    map (valOf o Int.fromString) ((cells o extract index o lines) s)
 
   fun column (s: string, index: int): int list =
-    let
-      fun extract (l: string list): string =
-        List.nth (l, index - 1)
-    in
-      map (valOf o Int.fromString o extract o cells) (lines s)
-    end
+    map (valOf o Int.fromString o extract index o cells) (lines s)
 end