diff --git a/byg/src/Function.hs b/byg/src/Function.hs
index f55f5dd..640e580 100644
--- a/byg/src/Function.hs
+++ b/byg/src/Function.hs
@@ -7,7 +7,7 @@ module Function
   , fileComponents
   , lowerString
   , elemOf
-  , elemOfString
+  , elemOf'
   , makeTemplate
   , applyTemplate
   , toText
@@ -79,10 +79,12 @@ lowerString :: TokenableTo String a => a -> DepGenM (Token String)
 lowerString a = runFunction LowerString =<< toToken a
 
 
-class (Show w, Lift w) => WitnessFor w t | w -> t
+class (Show w, Lift w) => WitnessFor w t | w -> t, t -> w where
+  witnessValue :: w
 
 data StringType = StringType deriving (Show, Lift)
-instance WitnessFor StringType String
+instance WitnessFor StringType String where
+  witnessValue = StringType
 
 data ElemOf a where ElemOf :: WitnessFor w a => w -> ElemOf a
 deriving instance Show (ElemOf a)
@@ -93,8 +95,8 @@ instance (Show a, Lift a, Valuable a, Eq a) => IsFunction (ElemOf a) (a, [a]) Bo
 elemOf :: (Show t, Lift t, Valuable t, Eq t, TokenableTo t a, TokenableTo [t] b, WitnessFor w t) => w -> a -> b -> DepGenM (Token Bool)
 elemOf witness a b = runFunction (ElemOf witness) =<< toTupleToken a b
 
-elemOfString :: (TokenableTo String a, TokenableTo [String] b) => a -> b -> DepGenM (Token Bool)
-elemOfString = elemOf StringType
+elemOf' :: (Show t, Lift t, Valuable t, Eq t, WitnessFor w t, TokenableTo t a, TokenableTo [t] b) => a -> b -> DepGenM (Token Bool)
+elemOf' = elemOf witnessValue
 
 
 data MakeTemplate = MakeTemplate deriving (Show, Lift)
diff --git a/byg/src/SiteGenerator.hs b/byg/src/SiteGenerator.hs
index 6c6f2de..1d55520 100644
--- a/byg/src/SiteGenerator.hs
+++ b/byg/src/SiteGenerator.hs
@@ -17,7 +17,7 @@ copyTo path targetDir = do
 hasExtension :: (TokenableTo [String] a, TokenableTo FilePath b) => a -> b -> DepGenM (Token Bool)
 hasExtension exts filename = do
   ext <- lowerString =<< untupleSndDepGenM =<< fileComponents filename
-  ext `elemOfString` exts
+  ext `elemOf'` exts
 
 handleRecipeDir :: Token FilePath -> Token Template -> Token FilePath -> Token FilePath -> DepGenM ()
 handleRecipeDir outputDir htmlTemplate indexName dir = do