Index: tests/natools-string_slice_set_tests.adb
==================================================================
--- tests/natools-string_slice_set_tests.adb
+++ tests/natools-string_slice_set_tests.adb
@@ -869,19 +869,37 @@
               ("First index is" & Integer'Image (Index)
                & ", expected" & Integer'Image (Parent_String'First));
             Dump (Report, Set);
             return;
          end if;
+
+         if Set.Previous (Index) /= 0 then
+            Report.Item (Name, NT.Fail);
+            Report.Info
+              ("Unexpected predecessor of First: "
+               & Integer'Image (Set.Previous (Index)));
+            Dump (Report, Set);
+            return;
+         end if;
 
          if Set_Last /= Parent_String'Last then
             Report.Item (Name, NT.Fail);
             Report.Info
               ("Last index is" & Integer'Image (Set_Last)
                & ", expected" & Integer'Image (Parent_String'Last));
             Dump (Report, Set);
             return;
          end if;
+
+         if Set.Next (Set_Last) /= 0 then
+            Report.Item (Name, NT.Fail);
+            Report.Info
+              ("Unexpected successor of Last: "
+               & Integer'Image (Set.Next (Set_Last)));
+            Dump (Report, Set);
+            return;
+         end if;
 
          loop
             if Set.Element (Index) /= Parent_String (Index) then
                Report.Item (Name, NT.Fail);
                Report.Info