diff --git a/src/compiler/GF/Grammar/Grammar.hs b/src/compiler/GF/Grammar/Grammar.hs
index 587b09a9f..448fa1657 100644
--- a/src/compiler/GF/Grammar/Grammar.hs
+++ b/src/compiler/GF/Grammar/Grammar.hs
@@ -78,6 +78,7 @@ import PGF.Internal (FId, FunId, SeqId, LIndex, Sequence, BindType(..))
 import Data.Array.IArray(Array)
 import Data.Array.Unboxed(UArray)
 import qualified Data.Map as Map
+import qualified Data.Set as Set
 import GF.Text.Pretty
 
 
@@ -125,10 +126,20 @@ extends :: ModuleInfo -> [ModuleName]
 extends = map fst . mextend
 
 isInherited :: MInclude -> Ident -> Bool
-isInherited c i = case c of
-  MIAll -> True
-  MIOnly is -> elem i is
-  MIExcept is -> notElem i is
+isInherited c =
+  case c of
+    MIAll -> const True
+    MIOnly is -> elemOrd is
+    MIExcept is -> not . elemOrd is
+
+-- | Faster version of `elem`, using a `Set`.
+-- Make sure you give this the first argument _outside_ of the inner loop
+--
+-- Example:
+-- > myIntersection xs ys = filter (elemOrd xs) ys
+elemOrd :: Ord a => [a] -> a -> Bool
+elemOrd list = (`Set.member` set)
+  where set = Set.fromList list
 
 inheritAll :: ModuleName -> (ModuleName,MInclude)
 inheritAll i = (i,MIAll)