diff --git a/bs/TagEngine.bs b/bs/TagEngine.bs
index 8b12b9d..7726811 100644
--- a/bs/TagEngine.bs
+++ b/bs/TagEngine.bs
@@ -1,7 +1,6 @@
 package TagEngine(
-    MkTagType,
+    MkTagType(..),
     TagEngine(..),
-    Util.BasicResult(..),
     mkTagEngine) where
 
 import Vector
diff --git a/bs/Tests/TagEngineTester.bs b/bs/Tests/TagEngineTester.bs
index 4fc700e..5a26477 100644
--- a/bs/Tests/TagEngineTester.bs
+++ b/bs/Tests/TagEngineTester.bs
@@ -2,6 +2,10 @@ package TagEngineTester(mkTagEngineTester) where
 
 import TagEngine
 import ActionSeq
+import Printf
+import Util
+
+type NumTags = 5
 
 mkTagEngineTester :: Module Empty
 mkTagEngineTester = do
@@ -26,39 +30,44 @@ mkTagEngineTester = do
 
     in
       actionSeq $
-      do requestTagAction
-      |> do requestTagAction
-      |> do requestTagAction
-      |> do requestTagAction
-      |> do requestTagAction
+      do
+        let expectedTag = 4
+        tag <- tagEngine.requestTag
+        dynamicAssert (tag == expectedTag) ""
+      |> do
+        let expectedTag = 3
+        tag <- tagEngine.requestTag
+        dynamicAssert (tag == expectedTag) ""
+      |> do
+        let expectedTag = 2
+        tag <- tagEngine.requestTag
+        dynamicAssert (tag == expectedTag) ""
+      |> do
+        let expectedTag = 1
+        tag <- tagEngine.requestTag
+        dynamicAssert (tag == expectedTag) ""
+      |> do
+        let expectedTag = 0
+        tag <- tagEngine.requestTag
+        dynamicAssert (tag == expectedTag) ""
       |> do retireTagAction 2
-      -- |> do $display "BEGIN TRY SIMULTANEOUS RETIRE and REQUEST"
       |> do
-          retireTagAction 4
-          requestTagAction
-      -- |> do $display "END TRY SIMULTANEOUS RETIRE and REQUEST"
-      -- |> do $display "BEGIN TRY SIMULTANEOUS RETIRE and REQUEST"
+        retireTagAction 4
+        let expectedTag :: MkTagType NumTags
+            expectedTag = 2
+        tag <- tagEngine.requestTag
+        dynamicAssert (tag == expectedTag) ""
       |> do
-          retireTagAction 4
-          requestTagAction
-      -- |> do $display "END TRY SIMULTANEOUS RETIRE and REQUEST"
-      |> do $finish
-      -- |> do retireTagAction 4
-      -- |> do retireTagAction 4
-      -- |> do retireTagAction 0
-      -- |> do requestTagAction
-      -- |> do requestTagAction
-      -- |> do retireTagAction 1
-      -- |> do requestTagAction
-      -- |> do $finish
+        retireTagAction 2
+        let expectedTag = 3
+        tag <- tagEngine.requestTag
+        dynamicAssert (tag == expectedTag) ""
+      |> do
+        terminateSimNoError
 
   addRules $
     rules
-      -- "counter": when True ==>
-      --   do
-      --     count := count + 1
-      --     $display "count : " (fshow count)
-      "testIncrement": when (runOnce == False) ==>
+      "start unit test": when (runOnce == False) ==>
         do
           s.start
           runOnce := True
diff --git a/bs/Top.bs b/bs/Top.bs
index a0816a0..b5276fd 100644
--- a/bs/Top.bs
+++ b/bs/Top.bs
@@ -7,9 +7,9 @@ import BRAM
 import CBindings
 import Bus
 import TagEngine
+
 import List
 import ActionSeq
-
 import Vector
 import BusTypes
 
diff --git a/bs/Util.bs b/bs/Util.bs
index 89492f5..0448782 100644
--- a/bs/Util.bs
+++ b/bs/Util.bs
@@ -1,24 +1,37 @@
 package Util(
   (|>),
-  BasicResult(..),
-  simulate_for) where
+  terminateSimNoError,
+  terminateSimWithError,
+  dynamicAssert
+  ) where
+
+import CBindings
 
 infixr 0 |>
 
-data BasicResult = Success
-                 | Failure
-                 deriving (Bits, Eq, FShow)
-
 (|>) :: (a -> b) -> a -> b
 f |> x = f x;
 
-simulate_for :: (Bits a n, Arith a, Eq a) => Reg a -> Reg a -> Rules
-simulate_for curr_cycle end_cycle =
-  rules
-    "count_cycle_rule": when True ==> action
-      curr_cycle := curr_cycle + 1
-      if curr_cycle == end_cycle
-        then
-          $finish
-        else
-          $display "cycle = " curr_cycle
+assertMessage :: String -> String -> String
+assertMessage which mess =
+  letseq pos = getStringPosition mess -- get position of original message literal
+  in (which +++" assertion failed: " +++ printPosition pos +++ mess)
+
+dynamicAssert :: Bool -> String -> Action
+dynamicAssert = if not testAssert then (\ _ _ -> noAction)
+                else (\ b s -> if b then noAction else
+                        action
+                          $display (assertMessage "Dynamic" s)
+                          restoreTerminal
+                          $finish 1
+                     )
+
+terminateSimNoError :: Action
+terminateSimNoError = do
+  restoreTerminal
+  $finish 0
+
+terminateSimWithError :: Action
+terminateSimWithError = do
+  restoreTerminal
+  $finish 1