diff --git a/current-versions.txt b/current-versions.txt
--- a/current-versions.txt
+++ b/current-versions.txt
@@ -1,7 +1,7 @@
 This package was tested to work with these dependency
 versions and compiler version.
 These are the default versions fetched by cabal install.
-Tested as of: 2014-02-24 04:37:13.583324 UTC
+Tested as of: 2014-03-01 19:44:29.719081 UTC
 Path to compiler: ghc-7.6.3
 Compiler description: 7.6.3
 
@@ -32,12 +32,16 @@
     time-1.4.0.1
     unix-2.6.0.1
 
-/home/massysett/prednote/sunlight-27776/db:
+/home/massysett/prednote/sunlight-17262/db:
     QuickCheck-2.6
-    prednote-0.18.0.4
+    contravariant-0.4.4
+    prednote-0.20.0.0
     rainbow-0.6.0.4
     random-1.0.1.1
     split-0.2.2
+    tagged-0.7
     terminfo-0.4.0.0
     text-1.1.0.0
+    transformers-0.3.0.0
+    transformers-compat-0.1.1.1
 
diff --git a/lib/Data/Prednote/Expressions.hs b/lib/Data/Prednote/Expressions.hs
--- a/lib/Data/Prednote/Expressions.hs
+++ b/lib/Data/Prednote/Expressions.hs
@@ -15,6 +15,7 @@
   ) where
 
 import Data.Either (partitionEithers)
+import Data.Functor.Contravariant
 import qualified Data.Text as X
 import qualified Data.Prednote.Expressions.Infix as I
 import qualified Data.Prednote.Expressions.RPN as R
@@ -22,6 +23,10 @@
 
 -- | A single type for both RPN tokens and infix tokens.
 newtype Token a = Token { unToken :: I.InfixToken a }
+
+instance Contravariant Token where
+  contramap f = Token . contramap f . unToken
+
 type Error = X.Text
 
 -- | Creates Operands from Pdct.
diff --git a/lib/Data/Prednote/Expressions/Infix.hs b/lib/Data/Prednote/Expressions/Infix.hs
--- a/lib/Data/Prednote/Expressions/Infix.hs
+++ b/lib/Data/Prednote/Expressions/Infix.hs
@@ -4,12 +4,18 @@
   , createRPN
   ) where
 
+import Data.Functor.Contravariant
 import qualified Data.Prednote.Expressions.RPN as R
 import qualified Data.Foldable as Fdbl
 
 data InfixToken a
   = TokRPN (R.RPNToken a)
   | TokParen Paren
+
+instance Contravariant InfixToken where
+  contramap f t = case t of
+    TokRPN r -> TokRPN . contramap f $ r
+    TokParen p -> TokParen p
 
 data Paren = Open | Close
 
diff --git a/lib/Data/Prednote/Expressions/RPN.hs b/lib/Data/Prednote/Expressions/RPN.hs
--- a/lib/Data/Prednote/Expressions/RPN.hs
+++ b/lib/Data/Prednote/Expressions/RPN.hs
@@ -6,6 +6,7 @@
 -- where @and@ and @or@ are binary and @not@ is unary.
 module Data.Prednote.Expressions.RPN where
 
+import Data.Functor.Contravariant
 import qualified Data.Foldable as Fdbl
 import qualified Data.Prednote.Pdct as P
 import Data.Prednote.Pdct ((&&&), (|||))
@@ -19,6 +20,11 @@
 data RPNToken a
   = TokOperand (P.Pdct a)
   | TokOperator Operator
+
+instance Contravariant RPNToken where
+  contramap f t = case t of
+    TokOperand p -> TokOperand . contramap f $ p
+    TokOperator o -> TokOperator o
 
 data Operator
   = OpAnd
diff --git a/lib/Data/Prednote/Pdct.hs b/lib/Data/Prednote/Pdct.hs
--- a/lib/Data/Prednote/Pdct.hs
+++ b/lib/Data/Prednote/Pdct.hs
@@ -23,8 +23,6 @@
   , (|||)
   , always
   , never
-  , boxPdct
-  , boxNode
 
   -- * Controlling whether Pdct are shown in the results
   , hide
@@ -77,6 +75,7 @@
 
 -- # Imports
 
+import Data.Functor.Contravariant
 import Data.Text (Text)
 import qualified Data.Text as X
 import Data.Monoid ((<>), mconcat, mempty)
@@ -176,25 +175,15 @@
 (|||) x y = Pdct "or" (const False) (Or [x, y])
 infixr 2 |||
 
--- | Given a function that un-boxes values of type b, changes a Pdct
--- from type a to type b.
-boxPdct
-  :: (b -> a)
-  -> Pdct a
-  -> Pdct b
-boxPdct f (Pdct l d n) = Pdct l d $ boxNode f n
+instance Contravariant Pdct where
+  contramap f (Pdct l d n) = Pdct l d $ contramap f n
 
--- | Given a function that un-boxes values of type b, changes a Node
--- from type a to type b.
-boxNode
-  :: (b -> a)
-  -> Node a
-  -> Node b
-boxNode f n = case n of
-  And ls -> And $ map (boxPdct f) ls
-  Or ls -> Or $ map (boxPdct f) ls
-  Not o -> Not $ boxPdct f o
-  Operand g -> Operand $ \b -> g (f b)
+instance Contravariant Node where
+  contramap f n = case n of
+    And ls -> And $ map (contramap f) ls
+    Or ls -> Or $ map (contramap f) ls
+    Not o -> Not $ contramap f o
+    Operand g -> Operand $ \b -> g (f b)
 
 -- # Result
 
diff --git a/lib/Data/Prednote/Test.hs b/lib/Data/Prednote/Test.hs
--- a/lib/Data/Prednote/Test.hs
+++ b/lib/Data/Prednote/Test.hs
@@ -24,6 +24,8 @@
 
   ) where
 
+import Control.Arrow (first)
+import Data.Functor.Contravariant
 import Data.Maybe (fromMaybe)
 import Data.Monoid ((<>), mempty)
 import qualified Data.Text as X
@@ -94,12 +96,18 @@
   -- ^ Default verbosity for the test.
   }
 
+instance Contravariant Test where
+  contramap f t = t { testFunc = testFunc t . f }
+
 data TestResult a = TestResult
   { resultName :: Name
   , resultPass :: Pass
   , resultSubjects :: [(a, Pt.Result)]
   , resultDefaultVerbosity :: TestVerbosity
   }
+
+instance Functor TestResult where
+  fmap f t = t { resultSubjects = map (first f) . resultSubjects $ t }
 
 -- # Showing tests
 
diff --git a/minimum-versions.txt b/minimum-versions.txt
--- a/minimum-versions.txt
+++ b/minimum-versions.txt
@@ -1,7 +1,7 @@
 This package was tested to work with these dependency
 versions and compiler version.
 These are the minimum versions given in the .cabal file.
-Tested as of: 2014-02-24 04:37:13.583324 UTC
+Tested as of: 2014-03-01 19:44:29.719081 UTC
 Path to compiler: ghc-7.4.1
 Compiler description: 7.4.1
 
@@ -33,12 +33,14 @@
     time-1.4
     unix-2.5.1.0
 
-/home/massysett/prednote/sunlight-27776/db:
+/home/massysett/prednote/sunlight-17262/db:
     QuickCheck-2.6
-    prednote-0.18.0.4
+    contravariant-0.2.0.1
+    prednote-0.20.0.0
     rainbow-0.6.0.4
     random-1.0.1.1
     split-0.2.2
     terminfo-0.4.0.0
     text-0.11.2.0
+    transformers-0.3.0.0
 
diff --git a/prednote.cabal b/prednote.cabal
--- a/prednote.cabal
+++ b/prednote.cabal
@@ -1,5 +1,5 @@
 name:                prednote
-version:             0.18.0.4
+version:             0.20.0.0
 synopsis:            Build and evaluate trees of predicates
 description:
   Build and evaluate trees of predicates. For example, you might build
@@ -41,6 +41,7 @@
 
   build-depends:
       base >= 4.5.0.0 && < 5
+    , contravariant >= 0.2.0.1
     , rainbow >=0.6.0.4
     , split >=0.2.2
     , text >= 0.11.2.0
@@ -53,11 +54,9 @@
   main-is: prednote-test.hs
   hs-source-dirs: . lib
 
-  -- Be sure the build-depends are listed within the if block;
-  -- otherwise, cabal install will always include these
-  -- build-dependencies in any build, even non-test builds.
   build-depends:
       base >=4.5.0.0 && < 5
+    , contravariant >= 0.2.0.1
     , QuickCheck >=2.6
     , rainbow >=0.6.0.4
     , text >= 0.11.2.0