1 files changed, 10 insertions, 0 deletions
diff --git a/lib/ur/listPair.ur b/lib/ur/listPair.ur
new file mode 100644
index 00000000..9a56f75a
--- /dev/null
+++ b/lib/ur/listPair.ur
@@ -0,0 +1,10 @@
+fun mapX (a ::: Type) (b ::: Type) (ctx ::: {Unit}) f =
+    let
+        fun mapX' ls1 ls2 =
+            case (ls1, ls2) of
+                ([], []) => <xml/>
+              | (x1 :: ls1, x2 :: ls2) => <xml>{f x1 x2}{mapX' ls1 ls2}</xml>
+              | _ => error <xml>ListPair.mapX: Unequal list lengths</xml>
+    in
+        mapX'
+    end