diff --git a/synthesizer-core.cabal b/synthesizer-core.cabal
--- a/synthesizer-core.cabal
+++ b/synthesizer-core.cabal
@@ -1,5 +1,5 @@
 Name:           synthesizer-core
-Version:        0.8.1.1
+Version:        0.8.1.2
 License:        GPL
 License-File:   LICENSE
 Author:         Henning Thielemann <haskell@henning-thielemann.de>
@@ -38,7 +38,7 @@
 
 
 Source-Repository this
-  Tag:         0.8.1.1
+  Tag:         0.8.1.2
   Type:        darcs
   Location:    http://code.haskell.org/synthesizer/core/
 
@@ -232,6 +232,7 @@
     storablevector,
     storable-tuple,
     event-list,
+    transformers,
     non-empty >=0.2.1 && <0.4,
     non-negative,
     utility-ht,
diff --git a/test/Test/Sound/Synthesizer/Plain/Control.hs b/test/Test/Sound/Synthesizer/Plain/Control.hs
--- a/test/Test/Sound/Synthesizer/Plain/Control.hs
+++ b/test/Test/Sound/Synthesizer/Plain/Control.hs
@@ -2,7 +2,8 @@
 
 import qualified Synthesizer.Plain.Control as Control
 
-import Test.QuickCheck (quickCheck, Property, (==>))
+import qualified Test.QuickCheck as QC
+import Test.QuickCheck (Property, quickCheck, (==>), )
 import Test.Utility (equalList, approxEqualListAbs, approxEqualListRel, )
 
 import Data.List (transpose)
@@ -46,33 +47,18 @@
       Control.linearStable d y0 :
       []
 
-{-
-Plain.Control.exponential: Falsifiable, after 88 tests:
--8.333333333333326e-2
-3.375
-
-Plain.Control.exponential: Falsifiable, after 69 tests:
-9.090909090909083e-2
--10.0
-
-Plain.Control.exponential: Falsifiable, after 73 tests:
--0.125
--1.1428571428571428
-
-Plain.Control.exponential2: Falsifiable, after 33 tests:
--7.692307692307687e-2
-9.5
--}
-exponential :: Double -> Double -> Bool
-exponential time y0 =
+exponential :: Double -> Property
+exponential y0 =
+   QC.forAll (QC.choose (10,1000)) $ \time ->
    all (approxEqualListRel (1e-10)) $ take 100 $ transpose $
       Control.exponential time y0 :
       Control.exponentialMultiscale time y0 :
       Control.exponentialStable time y0 :
       []
 
-exponential2 :: Double -> Double -> Bool
-exponential2 time y0 =
+exponential2 :: Double -> Property
+exponential2 y0 =
+   QC.forAll (QC.choose (10,1000)) $ \time ->
    all (approxEqualListRel (1e-10)) $ take 100 $ transpose $
       Control.exponential2 time y0 :
       Control.exponential2Multiscale time y0 :
diff --git a/test/Test/Sound/Synthesizer/Plain/Filter/Allpass.hs b/test/Test/Sound/Synthesizer/Plain/Filter/Allpass.hs
--- a/test/Test/Sound/Synthesizer/Plain/Filter/Allpass.hs
+++ b/test/Test/Sound/Synthesizer/Plain/Filter/Allpass.hs
@@ -1,7 +1,9 @@
 module Test.Sound.Synthesizer.Plain.Filter.Allpass (tests) where
 
 import qualified Synthesizer.Plain.Filter.Recursive.Allpass as Allpass
--- import qualified Synthesizer.Plain.Signal as Sig
+import qualified Synthesizer.Plain.Signal as Sig
+
+import qualified Number.NonNegative as NonNeg
 
 -- import qualified Test.Sound.Synthesizer.Plain.NonEmpty as NonEmpty