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diff --git a/users/aspen/xanthous/test/Xanthous/UtilSpec.hs b/users/aspen/xanthous/test/Xanthous/UtilSpec.hs
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+module Xanthous.UtilSpec (main, test) where
+
+import Test.Prelude
+import Xanthous.Util
+import Control.Monad.State.Lazy (execState)
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Util"
+  [ testGroup "smallestNotIn"
+    [ testCase "examples" $ do
+        smallestNotIn [7 :: Word, 3, 7] @?= 0
+        smallestNotIn [7 :: Word, 0, 1, 3, 7] @?= 2
+    , testProperty "returns an element not in the list" $ \(xs :: [Word]) ->
+        smallestNotIn xs `notElem` xs
+    , testProperty "pred return is in the list" $ \(xs :: [Word]) ->
+        let res = smallestNotIn xs
+        in res /= 0 ==> pred res `elem` xs
+    , testProperty "ignores order" $ \(xs :: [Word]) ->
+        forAll (shuffle xs) $ \shuffledXs ->
+          smallestNotIn xs === smallestNotIn shuffledXs
+    ]
+  , testGroup "takeWhileInclusive"
+    [ testProperty "takeWhileInclusive (const True) ≡ id"
+      $ \(xs :: [Int]) -> takeWhileInclusive (const True) xs === xs
+    ]
+  , testGroup "endoTimes"
+    [ testCase "endoTimes 4 succ 5"
+      $ endoTimes (4 :: Int) succ (5 :: Int) @?= 9
+    ]
+  , testGroup "modifyKL"
+    [ testCase "_1 += 1"
+      $ execState (modifyKL _1 $ pure . succ) (1 :: Int, 2 :: Int) @?= (2, 2)
+    ]
+  , testGroup "removeFirst"
+    [ testCase "example" $
+      removeFirst @[Int] (> 5) [1..10] @?= [1, 2, 3, 4, 5, 7, 8, 9, 10]
+    , testProperty "the result is the right length" $ \(xs :: [Int]) p ->
+        length (removeFirst p xs) `elem` [length xs, length xs - 1]
+    ]
+  , testGroup "AlphaChar"
+    [ testCase "succ 'z'" $ succ (AlphaChar 'z') @?= AlphaChar 'A'
+    ]
+  ]