diff --git a/Simulation/Aivika/Dynamics/Agent.hs b/Simulation/Aivika/Dynamics/Agent.hs
--- a/Simulation/Aivika/Dynamics/Agent.hs
+++ b/Simulation/Aivika/Dynamics/Agent.hs
@@ -288,10 +288,10 @@
 
 -- | Return a signal that notifies about every change of the state.
 agentStateChanged :: Agent -> Signal (Maybe AgentState)
-agentStateChanged agent = merge2Signals m1 m2
-  where m1 = publishSignal (agentStateChangedSource agent)
-        m2 = publishSignal (agentStateUpdatedSource agent)
-        
+agentStateChanged v = merge2Signals m1 m2    -- N.B. The order is important!
+  where m1 = publishSignal (agentStateUpdatedSource v)
+        m2 = publishSignal (agentStateChangedSource v)
+
 -- | Return a signal that notifies about every change of the state.
 agentStateChanged_ :: Agent -> Signal ()
 agentStateChanged_ agent =
diff --git a/Simulation/Aivika/Dynamics/Internal/Signal.hs b/Simulation/Aivika/Dynamics/Internal/Signal.hs
--- a/Simulation/Aivika/Dynamics/Internal/Signal.hs
+++ b/Simulation/Aivika/Dynamics/Internal/Signal.hs
@@ -28,12 +28,14 @@
         apSignal,
         filterSignal,
         filterSignalM,
+        emptySignal,
         merge2Signals,
         merge3Signals,
         merge4Signals,
         merge5Signals) where
 
 import Data.IORef
+import Data.Monoid
 
 import Control.Monad
 import Control.Monad.Trans
@@ -54,7 +56,7 @@
 -- | The signal that can have disposable handlers.  
 data Signal a =
   Signal { handleSignal :: (a -> Dynamics ()) -> 
-                           Simulation (Simulation ()),
+                           Dynamics (Dynamics ()),
            -- ^ Subscribe the handler to the specified 
            -- signal and return a nested computation 
            -- that, being applied, unsubscribes the 
@@ -76,7 +78,7 @@
 
 -- | Subscribe the handler to the specified signal.
 -- To subscribe the disposable handlers, use function 'handleSignal'.
-handleSignal_ :: Signal a -> (a -> Dynamics ()) -> Simulation ()
+handleSignal_ :: Signal a -> (a -> Dynamics ()) -> Dynamics ()
 handleSignal_ signal h = 
   do x <- handleSignal signal h
      return ()
@@ -94,13 +96,16 @@
          source = SignalSource { publishSignal = signal, 
                                  triggerSignal = trigger }
          handle h =
-           Simulation $ \r ->
-           do x <- enqueueSignalHandler queue h
-              return $ liftIO $ dequeueSignalHandler queue x
+           Dynamics $ \p ->
+           do invokeDynamics p update
+              x <- enqueueSignalHandler queue h
+              return $ 
+                Dynamics $ \p ->
+                do invokeDynamics p update
+                   dequeueSignalHandler queue x
          trigger a =
            Dynamics $ \p ->
-           do let Dynamics m = update 
-              m p
+           do invokeDynamics p update 
               let h = queueStart queue
               triggerSignalHandlers h a p
      return source
@@ -118,7 +123,7 @@
          source = SignalSource { publishSignal = signal, 
                                  triggerSignal = trigger }
          handle h =
-           Simulation $ \r ->
+           Dynamics $ \p ->
            do x <- enqueueSignalHandler queue h
               return $ liftIO $ dequeueSignalHandler queue x
          trigger a =
@@ -192,6 +197,24 @@
                    writeIORef (handlerPrev y) prev
                    writeIORef (handlerNext x) next
 
+instance Functor Signal where
+  fmap = mapSignal
+  
+instance Monoid (Signal a) where 
+  
+  mempty = emptySignal
+  
+  mappend = merge2Signals
+  
+  mconcat [] = emptySignal
+  mconcat [x1] = x1
+  mconcat [x1, x2] = merge2Signals x1 x2
+  mconcat [x1, x2, x3] = merge3Signals x1 x2 x3
+  mconcat [x1, x2, x3, x4] = merge4Signals x1 x2 x3 x4
+  mconcat [x1, x2, x3, x4, x5] = merge5Signals x1 x2 x3 x4 x5
+  mconcat (x1 : x2 : x3 : x4 : x5 : xs) = 
+    mconcat $ merge5Signals x1 x2 x3 x4 x5 : xs
+  
 -- | Map the signal according the specified function.
 mapSignal :: (a -> b) -> Signal a -> Signal b
 mapSignal f m =
@@ -200,9 +223,6 @@
            updateSignal = 
              updateSignal m }
 
-instance Functor Signal where
-  fmap = mapSignal
-  
 -- | Filter only those signal values that satisfy to 
 -- the specified predicate.
 filterSignal :: (a -> Bool) -> Signal a -> Signal a
@@ -297,3 +317,13 @@
             handleSignal m $ \a -> do { x <- f; h (x a) },
            updateSignal =
              updateSignal m }
+
+-- | An empty signal which is never triggered.
+emptySignal :: Signal a
+emptySignal =
+  Signal { handleSignal = \h -> return $ return (),
+           updateSignal = return () }
+  
+invokeDynamics :: Point -> Dynamics a -> IO a
+{-# INLINE invokeDynamics #-}
+invokeDynamics p (Dynamics m) = m p
diff --git a/Simulation/Aivika/Dynamics/Ref.hs b/Simulation/Aivika/Dynamics/Ref.hs
--- a/Simulation/Aivika/Dynamics/Ref.hs
+++ b/Simulation/Aivika/Dynamics/Ref.hs
@@ -75,10 +75,9 @@
 
 -- | Return a signal that notifies about every change of the reference state.
 refChanged :: Ref a -> Signal a
-refChanged r = merge2Signals m1 m2
-  where
-    m1 = publishSignal (refChangedSource r)
-    m2 = publishSignal (refUpdatedSource r)
+refChanged v = merge2Signals m1 m2    -- N.B. The order is important!
+  where m1 = publishSignal (refUpdatedSource v)
+        m2 = publishSignal (refChangedSource v)
 
 -- | Return a signal that notifies about every change of the reference state.
 refChanged_ :: Ref a -> Signal ()
diff --git a/Simulation/Aivika/Dynamics/Signal.hs b/Simulation/Aivika/Dynamics/Signal.hs
--- a/Simulation/Aivika/Dynamics/Signal.hs
+++ b/Simulation/Aivika/Dynamics/Signal.hs
@@ -32,6 +32,7 @@
         apSignal,
         filterSignal,
         filterSignalM,
+        emptySignal,
         merge2Signals,
         merge3Signals,
         merge4Signals,
@@ -60,7 +61,7 @@
 -- | Create a new signal source when the state depends on the event queue.
 newSignalSource :: EventQueue -> Simulation (SignalSource a)
 newSignalSource queue = 
-  newSignalSourceWithUpdate $ runQueue queue
+  newSignalSourceWithUpdate $ runQueueSync queue
 
 -- | Await the signal.
 awaitSignal :: Signal a -> Process a
@@ -69,7 +70,7 @@
   Cont $ \c ->
   Dynamics $ \p ->
   do r <- newIORef Nothing
-     let Simulation m = 
+     let Dynamics m = 
            handleSignal signal $ 
            \a -> Dynamics $ 
                  \p -> do x <- readIORef r
@@ -77,11 +78,11 @@
                             Nothing ->
                               error "The signal was lost: awaitSignal."
                             Just x ->
-                              do let Simulation m = x
-                                 m $ pointRun p
+                              do let Dynamics m = x
+                                 m p
                                  let Dynamics m = resumeContByParams c a
                                  m p
-     h <- m $ pointRun p
+     h <- m p
      writeIORef r $ Just h
           
 -- | Represents the history of the signal values.
@@ -92,7 +93,7 @@
                   signalHistoryValues :: V.Vector a }
 
 -- | Create a history of the signal values.
-newSignalHistory :: Signal a -> Simulation (SignalHistory a)
+newSignalHistory :: Signal a -> Dynamics (SignalHistory a)
 newSignalHistory signal =
   do ts <- liftIO UV.newVector
      xs <- liftIO V.newVector
diff --git a/Simulation/Aivika/Dynamics/UVar.hs b/Simulation/Aivika/Dynamics/UVar.hs
--- a/Simulation/Aivika/Dynamics/UVar.hs
+++ b/Simulation/Aivika/Dynamics/UVar.hs
@@ -146,9 +146,9 @@
      
 -- | Return a signal that notifies about every change of the variable state.
 uvarChanged :: UVar a -> Signal a
-uvarChanged v = merge2Signals m1 m2
-  where m1 = publishSignal (uvarChangedSource v)
-        m2 = publishSignal (uvarUpdatedSource v)
+uvarChanged v = merge2Signals m1 m2    -- N.B. The order is important!
+  where m1 = publishSignal (uvarUpdatedSource v)
+        m2 = publishSignal (uvarChangedSource v)
 
 -- | Return a signal that notifies about every change of the variable state.
 uvarChanged_ :: UVar a -> Signal ()
diff --git a/Simulation/Aivika/Dynamics/Var.hs b/Simulation/Aivika/Dynamics/Var.hs
--- a/Simulation/Aivika/Dynamics/Var.hs
+++ b/Simulation/Aivika/Dynamics/Var.hs
@@ -145,9 +145,9 @@
      
 -- | Return a signal that notifies about every change of the variable state.
 varChanged :: Var a -> Signal a
-varChanged v = merge2Signals m1 m2
-  where m1 = publishSignal (varChangedSource v)
-        m2 = publishSignal (varUpdatedSource v)
+varChanged v = merge2Signals m1 m2    -- N.B. The order is important!
+  where m1 = publishSignal (varUpdatedSource v)
+        m2 = publishSignal (varChangedSource v)
 
 -- | Return a signal that notifies about every change of the variable state.
 varChanged_ :: Var a -> Signal ()
diff --git a/aivika.cabal b/aivika.cabal
--- a/aivika.cabal
+++ b/aivika.cabal
@@ -1,5 +1,5 @@
 name:            aivika
-version:         0.4.1
+version:         0.4.2
 synopsis:        A multi-paradigm simulation library
 description:
     Aivika is a small simulation library that covers many paradigms.