diff --git a/test/persistent-vector-tests.sml b/test/persistent-vector-tests.sml
index 6dd3751..be7f5d6 100644
--- a/test/persistent-vector-tests.sml
+++ b/test/persistent-vector-tests.sml
@@ -45,19 +45,54 @@ struct
       loopInRange lst
     end
 
-  val initial =
-    let
-      val f = PersistentVector.append
-      val pv = PersistentVector.empty
+  val appendTests = describe "PersistentVector.append"
+    [ test "contains appended values in range" (fn _ =>
+        let
+          (* arrange *)
+          val f = PersistentVector.append
+          val pv = PersistentVector.empty
 
-      val pv = f (1, 3, pv)
-      val pv = f (5, 7, pv)
-      val pv = f (9, 13, pv)
-      val pv = f (19, 27, pv)
-      val pv = f (33, 33, pv)
-    in
-      pv
-    end
+          (* act *)
+          val pv = f (1, 3, pv)
+          val pv = f (5, 7, pv)
+          val pv = f (9, 13, pv)
+          val pv = f (19, 27, pv)
+          val pv = f (33, 33, pv)
 
-  val tests = []
+          (* assert *)
+          (* we split the list into several smaller lists
+          * and then concatenate at the end
+          * so that the formatter does not cause
+          * each list element to take its own line *)
+          val indicesInRange1 = [1, 2, 3, 5, 6, 7, 9]
+          val indicesInRange2 = [10, 11, 12, 13, 19, 20]
+          val indicesInRange3 = [21, 22, 23, 24, 25, 26, 27, 33]
+
+          val indicesInRange =
+            List.concat [indicesInRange1, indicesInRange2, indicesInRange3]
+        in
+          isInRange (indicesInRange, pv)
+        end)
+    , test "does not contain values in range that were not appended" (fn _ =>
+        let
+          (* arrange *)
+          val f = PersistentVector.append
+          val pv = PersistentVector.empty
+
+          (* act *)
+          val pv = f (1, 3, pv)
+          val pv = f (5, 7, pv)
+          val pv = f (9, 13, pv)
+          val pv = f (19, 27, pv)
+          val pv = f (33, 33, pv)
+
+          (* assert *)
+          val indicesNotInRange =
+            [0, 4, 8, 14, 15, 16, 17, 18, 28, 29, 30, 31, 32, 34, 35]
+        in
+          isNotInRange (indicesNotInRange, pv)
+        end)
+    ]
+
+  val tests = [appendTests]
 end