diff --git a/.travis.yml b/.travis.yml
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,13 +6,18 @@
 
   # Try installing some of the build-deps with apt-get for speed.
   - travis/cabal-apt-install $mode
+  - cabal install happy packunused packdeps
+  - export PATH=~/.cabal/bin:$PATH
 
 install:
   - cabal configure -flib-Werror $mode
   - cabal build
 
 script:
-  - $script && hlint src --cpp-define HLINT
+  - $script
+  - packdeps ersatz.cabal
+  - packunused
+  - hlint src --cpp-define HLINT
 
 notifications:
   irc:
diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+0.2.6.1
+-----
+* Exported `Ersatz.Variable.GVariable`
+
 0.2.6
 -----
 * `temporary 1.2` support
diff --git a/ersatz.cabal b/ersatz.cabal
--- a/ersatz.cabal
+++ b/ersatz.cabal
@@ -1,5 +1,5 @@
 name:           ersatz
-version:        0.2.6
+version:        0.2.6.1
 license:        BSD3
 license-file:   LICENSE
 author:         Edward A. Kmett, Johan Kiviniemi
@@ -170,7 +170,6 @@
     bytestring           >= 0.9     && < 0.12,
     containers           >= 0.2.0.1 && < 0.6,
     data-default         >= 0.5     && < 0.6,
-    ghc-prim,
     lens                 >= 3.8     && < 5,
     mtl                  >= 1.1     && < 2.2,
     process              >= 1.1     && < 1.3,
@@ -178,6 +177,9 @@
     transformers         == 0.3.*,
     unordered-containers == 0.2.*
 
+  if impl(ghc >= 7.4 && < 7.6)
+    build-depends: ghc-prim
+
   exposed-modules:
     Ersatz
     Ersatz.Bit
@@ -207,10 +209,11 @@
       base < 5,
       containers,
       ersatz,
-      ghc-prim,
       lens,
       mtl,
       parsec >= 3.1 && < 3.2
+    if impl(ghc >= 7.4 && < 7.6)
+      build-depends: ghc-prim
   else
     buildable: False
   main-is: Main.hs
@@ -229,8 +232,9 @@
       array,
       base < 5,
       ersatz,
-      ghc-prim,
       mtl
+    if impl(ghc >= 7.4 && < 7.6)
+      build-depends: ghc-prim
   else
     buildable: False
   default-language: Haskell2010
diff --git a/src/Ersatz/Bits.hs b/src/Ersatz/Bits.hs
--- a/src/Ersatz/Bits.hs
+++ b/src/Ersatz/Bits.hs
@@ -13,6 +13,7 @@
 --------------------------------------------------------------------
 module Ersatz.Bits
   ( Bit1(..), Bit2(..), Bit3(..), Bit4(..), Bit5(..), Bit6(..), Bit7(..), Bit8(..)
+  , full_adder, half_adder
   ) where
 
 import Prelude hiding ((&&), (||), and, or, not)
@@ -178,3 +179,41 @@
 boolToNum False = 0
 boolToNum True  = 1
 {-# INLINE boolToNum #-}
+
+instance Num Bit1 where
+  Bit1 a + Bit1 b = Bit1 (xor a b)
+  Bit1 a * Bit1 b = Bit1 (a && b)
+  Bit1 a - Bit1 b = Bit1 (xor a b)
+  negate a = a
+  abs a    = a
+  signum a = a
+  fromInteger = Bit1 . bool . odd
+
+full_adder :: Bit -> Bit -> Bit -> (Bit, Bit)
+full_adder a b cin = (s2, c1 || c2)
+  where (s1,c1) = half_adder a b
+        (s2,c2) = half_adder s1 cin
+
+half_adder :: Bit -> Bit -> (Bit, Bit)
+half_adder a b = (a `xor` b, a && b)
+
+instance Num Bit2 where
+  Bit2 a2 a1 + Bit2 b2 b1 = Bit2 s2 s1 where
+    (s1,c2) = half_adder a1 b1
+    (s2,_)  = full_adder a2 b2 c2
+  Bit2 a2 a1 * Bit2 b2 b1 = Bit2 ((a1 && b2) `xor` (a2 && b1)) (a1 && b1)
+    -- wallace tree
+    --
+    --   XX
+    --  XX
+    -- ----
+    --  XXX
+    --  X
+    -- ----
+    -- XXXX
+    --
+    -- But we only need the first 2 bits
+  negate (Bit2 a b) = Bit2 (not a) (not b) + 1
+  abs a = a
+  signum (Bit2 a b) = Bit2 false (a || b)
+  fromInteger k = Bit2 (bool (k .&. 2 /= 0)) (bool (k .&. 1 /= 0))
diff --git a/src/Ersatz/Variable.hs b/src/Ersatz/Variable.hs
--- a/src/Ersatz/Variable.hs
+++ b/src/Ersatz/Variable.hs
@@ -14,6 +14,7 @@
 --------------------------------------------------------------------
 module Ersatz.Variable
   ( Variable(..)
+  , GVariable(..)
   ) where
 
 import Control.Monad