src/Pure/library.ML

         | rep (n, xs) = rep (n - 1, x :: xs)
   in
     if n < 0 then raise LIST "replicate"
     else rep (n, [])
   end;
+
+(*multiply [a, b, c, ...] * [xs, ys, zs, ...]*)
+fun multiply ([], _) = []
+  | multiply (x :: xs, yss) = map (cons x) yss @ multiply (xs, yss);
 
 
 (* filter *)
 
 (*copy the list preserving elements that satisfy the predicate*)

 (* handle errors capturing messages *)
 
 datatype 'a error =
   Error of string |
   OK of 'a;
+
+fun get_error (Error msg) = Some msg
+  | get_error _ = None;
+
+fun get_ok (OK x) = Some x
+  | get_ok _ = None;
 
 fun handle_error f x =
   let
     val buffer = ref "";
     fun capture s = buffer := ! buffer ^ s ^ "\n";