add pending test

unison-src/tests/mergesort.uu

@@ -0,0 +1,33 @@
+
+-- > replicate 4 "a"
+-- > x = replicate 20 "a"
+
+-- current triggers runtime error
+-- java.lang.ClassCastException: org.unisonweb.Builtins$External$$anon$8 cannot be cast to org.unisonweb.Value$Lambda
+	at org.unisonweb.compilation.package$.org$unisonweb$compilation$package$$$anonfun$dynamicCall$1(compilation.scala:747) 
+> sort (<) [9234,23,1,3,6,2,3,51,24,1,3,55,2,1]
+
+halveWith : ([a] -> [a] -> b) -> [a] -> b
+halveWith k a = k (take (size a / 2) a) (drop (size a / 2) a)
+
+sort : (a -> a -> Boolean) -> [a] -> [a]
+sort lte a =
+  if size a < 2 then a
+  else halveWith (l r -> merge lte (sort lte l) (sort lte r)) a
+
+merge : (a -> a -> Boolean) -> [a] -> [a] -> [a]
+merge lte a b =
+  use Sequence ++
+  use Optional None Some
+  go acc a b = case at 0 a of
+    None -> acc ++ b
+    Some hd1 -> case at 0 b of
+      None -> acc ++ a
+      Some hd2 ->
+        if hd1 `lte` hd2 then
+          go (acc `snoc` hd1) (drop 1 a) b
+        else
+          go (acc `snoc` hd2) a (drop 1 b)
+  go [] a b
+
+()