diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,10 @@
+## 0.1.1
+
+- Add `IsString Word8Set` instance, with `fromString = fromASCII`.
+- Add `PartialOrd Word8Set` instance.
+- Add `Heyting Word8Set` instance.
+- Add `symmetricDifference`.
+
 ## 0.1
 
 * First version. Released on an unsuspecting world.
diff --git a/src/Data/Word8Set.hs b/src/Data/Word8Set.hs
--- a/src/Data/Word8Set.hs
+++ b/src/Data/Word8Set.hs
@@ -58,6 +58,7 @@
     union,
     unions,
     difference,
+    symmetricDifference,
     (\\),
     intersection,
     complement,
@@ -106,11 +107,14 @@
        otherwise, return, showParen, showString, snd, ($), (&&), (+), (-), (.), (/=), (<=), (>))
 
 import Algebra.Lattice       (BoundedJoinSemiLattice (..), BoundedMeetSemiLattice (..), Lattice (..))
+import Algebra.Heyting (Heyting (..))
+import Algebra.PartialOrd (PartialOrd (..))
 import Control.DeepSeq       (NFData (..))
 import Data.Bits             ((.&.), (.|.))
 import Data.Char             (chr, ord)
 import Data.Monoid           (Monoid (..))
 import Data.Semigroup        (Semigroup (..))
+import Data.String           (IsString (..))
 import Data.WideWord.Word256 (Word256 (..))
 import Data.Word             (Word8)
 import Test.QuickCheck       (Arbitrary (..), CoArbitrary (..), Function (..), functionMap)
@@ -191,11 +195,31 @@
 instance BoundedMeetSemiLattice Word8Set where
     top = full
 
+-- |
+--
+-- @since 0.1.1
+instance Heyting Word8Set where
+    a ==> b = neg a \/ b 
+    neg     = complement
+    a <=> b = complement (symmetricDifference a b)
+
+-- | @'leq' = 'isSubsetOf'@
+--
+-- @since 0.1.1
+instance PartialOrd Word8Set where
+    leq = isSubsetOf
+
 instance GHC.Exts.IsList Word8Set where
   type Item Word8Set = Key
   fromList = fromList
   toList   = toList
 
+-- | @'fromString' = 'fromASCII'@
+--
+-- @since 0.1.1
+instance IsString Word8Set where
+    fromString = fromASCII
+
 -------------------------------------------------------------------------------
 -- Construction
 -------------------------------------------------------------------------------
@@ -494,6 +518,20 @@
 -- | Difference between two sets.
 difference :: Word8Set -> Word8Set -> Word8Set
 difference xs ys = intersection xs (complement ys)
+
+-- | Symmetric difference between two sets
+--
+-- @
+-- 'symmetricDifference' xs ys = 'difference' ('union' xs ys) ('intersection' xs ys)
+-- @
+--
+-- >>> symmetricDifference (range 10 20) (range 15 25)
+-- fromList [10,11,12,13,14,21,22,23,24,25]
+--
+-- @since 0.1.1
+--
+symmetricDifference :: Word8Set -> Word8Set -> Word8Set
+symmetricDifference (W8S xs) (W8S ys) = W8S (Bits.xor xs ys)
 
 -- | See 'difference'.
 (\\) :: Word8Set -> Word8Set -> Word8Set
diff --git a/tests/word8set-tests.hs b/tests/word8set-tests.hs
new file mode 100644
--- /dev/null
+++ b/tests/word8set-tests.hs
@@ -0,0 +1,46 @@
+module Main (main) where
+
+import Algebra.Heyting       (Heyting (..))
+import Algebra.Lattice       (bottom, top, (/\))
+import Data.Word8Set         (Word8Set)
+import Test.QuickCheck       (Property, (===))
+import Test.Tasty            (defaultMain, testGroup)
+import Test.Tasty.QuickCheck (testProperty)
+
+main :: IO ()
+main = defaultMain $ testGroup "word8set"
+    [ testProperty "heyting1" heyting1
+    , testProperty "heyting2" heyting2
+    , testProperty "heyting3" heyting3
+    , testProperty "heyting4" heyting4
+    , testProperty "heytingNeg" heytingNeg
+    , testProperty "heytingEquiv" heytingEquiv
+    ]
+
+-------------------------------------------------------------------------------
+-- Heyting
+--
+-- x ==> x        ≡ top
+-- x /\ (x ==> y) ≡ x /\ y
+-- y /\ (x ==> y) ≡ y
+-- x ==> (y /\ z) ≡ (x ==> y) /\ (x ==> z)
+--
+-------------------------------------------------------------------------------
+
+heyting1 :: Word8Set -> Property
+heyting1 x = (x ==> x) === top
+
+heyting2 :: Word8Set -> Word8Set -> Property
+heyting2 x y = (x /\ (x ==> y)) === x /\ y
+
+heyting3 :: Word8Set -> Word8Set -> Property
+heyting3 x y = (y /\ (x ==> y)) === y
+
+heyting4 :: Word8Set -> Word8Set -> Word8Set -> Property
+heyting4 x y z = (x ==> (y /\ z)) === (x ==> y) /\ (x ==> z)
+
+heytingNeg :: Word8Set -> Property
+heytingNeg x = neg x === x ==> bottom
+
+heytingEquiv :: Word8Set -> Word8Set -> Property
+heytingEquiv x y = x <=> y === (x ==> y) /\ (y ==> x)
diff --git a/word8set.cabal b/word8set.cabal
--- a/word8set.cabal
+++ b/word8set.cabal
@@ -1,6 +1,6 @@
 cabal-version:      3.0
 name:               word8set
-version:            0.1
+version:            0.1.1
 synopsis:           Word8 set
 description:        Set of Word8 numbers. Backed up by Word256.
 category:           Data
@@ -44,3 +44,21 @@
     , lattices    ^>=2.2
     , QuickCheck  ^>=2.14.2
     , wide-word   ^>=0.1.5.0
+
+test-suite word8set-tests
+  default-language: Haskell2010
+  type:             exitcode-stdio-1.0
+  hs-source-dirs:   tests
+  main-is:          word8set-tests.hs
+
+  -- library dependencies
+  build-depends:
+    , base
+    , lattices
+    , QuickCheck
+    , word8set
+
+  -- test dependencies
+  build-depends:
+    , tasty             ^>=1.4.3
+    , tasty-quickcheck  ^>=0.10.2