diff --git a/bearriver.cabal b/bearriver.cabal
--- a/bearriver.cabal
+++ b/bearriver.cabal
@@ -1,5 +1,5 @@
 name:                bearriver
-version:             0.10.4.2
+version:             0.10.4.3
 synopsis:            A replacement of Yampa based on Monadic Stream Functions.
 description:         A Yampa replacement built using Dunai.
 homepage:            keera.co.uk
@@ -7,10 +7,10 @@
 license-file:        LICENSE
 author:              Ivan Perez and Manuel Bärenz
 maintainer:          ivan.perez@keera.co.uk
--- copyright:           
+-- copyright:
 category:            FRP
 build-type:          Simple
--- extra-source-files:  
+-- extra-source-files:
 cabal-version:       >=1.10
 
 library
@@ -23,7 +23,7 @@
                        FRP.Yampa.AffineSpace,
                        FRP.BearRiver
 
-  build-depends:       base >=4.7 && <5, transformers >=0.3, mtl, dunai, MonadRandom
+  build-depends:       base >=4.6 && <5, transformers >=0.3, mtl, dunai, MonadRandom
   hs-source-dirs:      src/
   default-language:    Haskell2010
 
diff --git a/src/FRP/BearRiver.hs b/src/FRP/BearRiver.hs
--- a/src/FRP/BearRiver.hs
+++ b/src/FRP/BearRiver.hs
@@ -146,6 +146,24 @@
 loopPre :: Monad m => c -> SF m (a, c) (b, c) -> SF m a b
 loopPre = feedback
 
+-- | Event source that never occurs.
+never :: Monad m => SF m a (Event b)
+never = constant NoEvent
+
+-- | Event source with a single occurrence at time 0. The value of the event
+-- is given by the function argument.
+now :: Monad m => b -> SF m a (Event b)
+now b0 = Event b0 --> never
+
+-- | Suppress all but the first event.
+once :: Monad m => SF m (Event a) (Event a)
+once = takeEvents 1
+
+-- | Suppress all but the first n events.
+takeEvents :: Monad m => Int -> SF m (Event a) (Event a)
+takeEvents n | n <= 0 = never
+takeEvents n = dSwitch (arr dup) (const (NoEvent >-- takeEvents (n - 1)))
+
 after :: Monad m
       => Time -- ^ The time /q/ after which the event should be produced
       -> b    -- ^ Value to produce at that time
@@ -173,10 +191,24 @@
   timeDelta :: Monad m => SF m a DTime
   timeDelta = arrM_ ask
 
+-- | Initialization operator (cf. Lustre/Lucid Synchrone).
+--
+-- The output at time zero is the first argument, and from
+-- that point on it behaves like the signal function passed as
+-- second argument.
 (-->) :: Monad m => b -> SF m a b -> SF m a b
-b0 --> sf = MSF $ \a -> do
-  (_, ct) <- unMSF sf a
-  return (b0, ct)
+b0 --> sf = sf >>> replaceOnce b0
+
+-- | Input initialization operator.
+--
+-- The input at time zero is the first argument, and from
+-- that point on it behaves like the signal function passed as
+-- second argument.
+(>--) :: Monad m => a -> SF m a b -> SF m a b
+a0 >-- sf = replaceOnce a0 >>> sf
+
+replaceOnce :: Monad m => a -> SF m a a
+replaceOnce a = dSwitch (arr $ const (a, Event ())) (const $ arr id)
 
 accumHoldBy :: Monad m => (b -> a -> b) -> b -> SF m (Event a) b
 accumHoldBy f b = feedback b $ arr $ \(a, b') ->