diff --git a/src/GF/Data/Operations.hs b/src/GF/Data/Operations.hs
index 9bcae5c6a..377ac736f 100644
--- a/src/GF/Data/Operations.hs
+++ b/src/GF/Data/Operations.hs
@@ -56,7 +56,7 @@ module GF.Data.Operations (-- * misc functions
 		   sortByLongest, combinations, mkTextFile, initFilePath,
 
 		   -- * topological sorting with test of cyclicity
-		   topoTest, topoSort, cyclesIn,
+		   topoTest, 
 
 		   -- * the generic fix point iterator
 		   iterFix,
@@ -82,6 +82,7 @@ import Data.Map (Map)
 import Control.Monad (liftM,liftM2, MonadPlus, mzero, mplus)
 
 import GF.Data.ErrM
+import GF.Data.Relation
 
 infixr 5 +++
 infixr 5 ++-
@@ -477,36 +478,8 @@ initFilePath :: FilePath -> FilePath
 initFilePath f = reverse (dropWhile (/='/') (reverse f))
 
 -- | topological sorting with test of cyclicity
-topoTest :: Eq a => [(a,[a])] -> Either [a] [[a]]
-topoTest g = if length g' == length g then Left g' else Right (cyclesIn g ++[[]]) 
-  where
-    g' = topoSort g
-  
-cyclesIn :: Eq a => [(a,[a])] -> [[a]]
-cyclesIn deps = nubb $ clean $ filt $ iterFix findDep immediate where
-  immediate = [[y,x] | (x,xs) <- deps, y <- xs] 
-  findDep chains = [y:x:chain | 
-                      x:chain <- chains, (x',xs) <- deps, x' == x, y <- xs, 
-                      notElem y (init chain)]
-
-  clean = map remdup
-  nubb = nubBy (\x y -> y == reverse x) 
-  filt = filter (\xs -> last xs == head xs)
-  remdup (x:xs) = x : remdup xs' where xs' = dropWhile (==x) xs
-  remdup [] = []
-
-
--- | topological sorting 
-topoSort :: Eq a => [(a,[a])] -> [a]
-topoSort g = reverse $ tsort 0 [ffs | ffs@(f,_) <- g, inDeg f == 0] [] where
-  tsort _ []     r = r
-  tsort k (ffs@(f,fs) : cs) r
-    | elem f r  = tsort k cs r
-    | k > lx    = r  
-    | otherwise = tsort (k+1) cs (f : tsort (k+1) (info fs) r)
-  info hs = [(f,fs) | (f,fs) <- g, elem f hs]
-  inDeg f = length [t | (h,hs) <- g, t <- hs, t == f]
-  lx = length g
+topoTest :: Ord a => [(a,[a])] -> Either [a] [[a]]
+topoTest = topologicalSort . mkRel'
 
 -- | the generic fix point iterator
 iterFix :: Eq a => ([a] -> [a]) -> [a] -> [a]
diff --git a/src/GF/Data/Relation.hs b/src/GF/Data/Relation.hs
index 1a052ec68..7024a482c 100644
--- a/src/GF/Data/Relation.hs
+++ b/src/GF/Data/Relation.hs
@@ -22,12 +22,16 @@ module GF.Data.Relation (Rel, mkRel, mkRel'
                            , equivalenceClasses
                            , isTransitive, isReflexive, isSymmetric
                            , isEquivalence
-                           , isSubRelationOf) where
+                           , isSubRelationOf
+                           , topologicalSort) where
 
+import Data.Foldable (toList)
 import Data.List
 import Data.Maybe
 import Data.Map (Map)
 import qualified Data.Map as Map
+import Data.Sequence (Seq)
+import qualified Data.Sequence as Seq
 import Data.Set (Set)
 import qualified Data.Set as Set
 
@@ -44,7 +48,7 @@ mkRel ps = relates ps Map.empty
 mkRel' :: Ord a => [(a,[a])] -> Rel a
 mkRel' xs = Map.fromListWith Set.union [(x,Set.fromList ys) | (x,ys) <- xs]
 
-relToList :: Rel a -> [(a,a)]
+relToList :: Ord a => Rel a -> [(a,a)]
 relToList r = [ (x,y) | (x,ys) <- Map.toList r, y <- Set.toList ys ]
 
 -- | Add a pair to the relation.
@@ -67,6 +71,9 @@ allRelated r x = fromMaybe Set.empty (Map.lookup x r)
 domain :: Ord a => Rel a -> Set a
 domain r = foldl Set.union (Map.keysSet r) (Map.elems r)
 
+reverseRel :: Ord a => Rel a -> Rel a
+reverseRel r = mkRel [(y,x) | (x,y) <- relToList r]
+
 -- | Keep only pairs for which both elements are in the given set.
 intersectSetRel :: Ord a => Set a -> Rel a -> Rel a
 intersectSetRel s = filterRel (\x y -> x `Set.member` s && y `Set.member` s)
@@ -98,12 +105,12 @@ reflexiveElements r = Set.fromList [ x | (x,ys) <- Map.toList r, x `Set.member`
 
 -- | Keep the related pairs for which the predicate is true.
 filterRel :: Ord a => (a -> a -> Bool) -> Rel a -> Rel a 
-filterRel p = purgeEmpty . Map.mapWithKey (Set.filter . p)
+filterRel p = fst . purgeEmpty . Map.mapWithKey (Set.filter . p)
 
 -- | Remove keys that map to no elements.
-purgeEmpty :: Ord a => Rel a -> Rel a
-purgeEmpty r = Map.filter (not . Set.null) r
-
+purgeEmpty :: Ord a => Rel a -> (Rel a, Set a)
+purgeEmpty r = let (r',r'') = Map.partition (not . Set.null) r
+                in (r', Map.keysSet r'')
 
 -- | Get the equivalence classes from an equivalence relation. 
 equivalenceClasses :: Ord a => Rel a -> [Set a]
@@ -128,3 +135,59 @@ isEquivalence r = isReflexive r && isSymmetric r && isTransitive r
 
 isSubRelationOf :: Ord a => Rel a -> Rel a -> Bool
 isSubRelationOf r1 r2 = all (uncurry (isRelatedTo r2)) (relToList r1)
+
+-- | Returns 'Left' if there are cycles, and 'Right' if there are cycles.
+topologicalSort :: Ord a => Rel a -> Either [a] [[a]]
+topologicalSort r = tsort r' noIncoming Seq.empty
+  where r' = relToRel' r
+        noIncoming = Seq.fromList [x | (x,(is,_)) <- Map.toList r', Set.null is]
+
+tsort :: Ord a => Rel' a -> Seq a -> Seq a -> Either [a] [[a]]
+tsort r xs l = case Seq.viewl xs of
+                 Seq.EmptyL | isEmpty' r -> Left (toList l)
+                            | otherwise  -> Right (findCycles (rel'ToRel r))
+                 x Seq.:< xs -> tsort r' (xs Seq.>< Seq.fromList new) (l Seq.|> x)
+                     where (r',_,os) = remove x r
+                           new = [o | o <- Set.toList os, Set.null (incoming o r')]
+
+findCycles :: Ord a => Rel a -> [[a]]
+findCycles = map Set.toList . equivalenceClasses . reflexiveSubrelation . symmetricSubrelation . transitiveClosure
+
+--
+-- * Alternative representation that keeps both incoming and outgoing edges
+--
+
+-- | Keeps both incoming and outgoing edges.
+type Rel' a = Map a (Set a, Set a)
+
+isEmpty' :: Ord a => Rel' a -> Bool
+isEmpty' = Map.null
+
+relToRel' :: Ord a => Rel a -> Rel' a
+relToRel' r = Map.unionWith (\ (i,_) (_,o) -> (i,o)) ir or
+  where ir = Map.map (\s -> (s,Set.empty)) $ reverseRel r
+        or = Map.map (\s -> (Set.empty,s)) $ r
+
+rel'ToRel :: Ord a => Rel' a -> Rel a
+rel'ToRel = Map.map snd
+
+-- | Removes an element from a relation.
+-- Returns the new relation, and the set of incoming and outgoing edges
+-- of the removed element.
+remove :: Ord a => a -> Rel' a -> (Rel' a, Set a, Set a)
+remove x r = let (mss,r') = Map.updateLookupWithKey (\_ _ -> Nothing) x r
+              in case mss of
+                   -- element was not in the relation
+                   Nothing      -> (r', Set.empty, Set.empty)
+                   -- remove element from all incoming and outgoing sets
+                   -- of other elements
+                   Just (is,os) -> 
+                       let r''  = foldr (\i -> Map.adjust (\ (is',os') -> (is', Set.delete x os')) i) r'  $ Set.toList is
+                           r''' = foldr (\o -> Map.adjust (\ (is',os') -> (Set.delete x is', os')) o) r'' $ Set.toList os
+                        in (r''', is, os)
+
+incoming :: Ord a => a -> Rel' a -> Set a
+incoming x r = maybe Set.empty fst $ Map.lookup x r
+
+outgoing :: Ord a => a -> Rel' a -> Set a
+outgoing x r = maybe Set.empty snd $ Map.lookup x r
\ No newline at end of file
diff --git a/src/GF/Infra/Modules.hs b/src/GF/Infra/Modules.hs
index 3b9cf6b6a..fc319f6b3 100644
--- a/src/GF/Infra/Modules.hs
+++ b/src/GF/Infra/Modules.hs
@@ -87,13 +87,13 @@ data ModuleType i =
   | MTInstance i 
   | MTReuse (MReuseType i)
   | MTUnion (ModuleType i) [(i,[i])] -- ^ not meant to be recursive
-  deriving (Eq,Show)
+  deriving (Eq,Ord,Show)
 
 data MReuseType i = MRInterface i | MRInstance i i | MRResource i
-  deriving (Show,Eq)
+  deriving (Eq,Ord,Show)
 
 data MInclude i = MIAll | MIOnly [i] | MIExcept [i]
-  deriving (Show,Eq)
+  deriving (Eq,Ord,Show)
 
 extends :: Module i a -> [i]
 extends = map fst . extend
@@ -165,13 +165,13 @@ data MainConcreteSpec i = MainConcreteSpec {
 data OpenSpec i = 
    OSimple OpenQualif i 
  | OQualif OpenQualif i i
-  deriving (Eq,Show)
+  deriving (Eq,Ord,Show)
 
 data OpenQualif =
    OQNormal
  | OQInterface
  | OQIncomplete
-  deriving (Eq,Show)
+  deriving (Eq,Ord,Show)
 
 oSimple :: i -> OpenSpec i
 oSimple = OSimple OQNormal
@@ -182,7 +182,7 @@ oQualif = OQualif OQNormal
 data ModuleStatus = 
    MSComplete 
  | MSIncomplete 
-  deriving (Eq,Show)
+  deriving (Eq,Ord,Show)
 
 openedModule :: OpenSpec i -> i
 openedModule o = case o of
@@ -280,7 +280,7 @@ data IdentM i = IdentM {
   identM :: i ,
   typeM  :: ModuleType i
   }
-  deriving (Eq,Show)
+  deriving (Eq,Ord,Show)
 
 typeOfModule :: ModInfo i a -> ModuleType i
 typeOfModule mi = case mi of
@@ -402,12 +402,14 @@ isCompleteModule m = mstatus m == MSComplete && mtype m /= MTInterface
 
 
 -- | all abstract modules sorted from least to most dependent
-allAbstracts :: Eq i => MGrammar i a -> [i]
-allAbstracts gr = topoSort 
-  [(i,extends m) | (i,ModMod m) <- modules gr, mtype m == MTAbstract]
+allAbstracts :: (Ord i, Show i) => MGrammar i a -> [i]
+allAbstracts gr = 
+ case topoTest [(i,extends m) | (i,ModMod m) <- modules gr, mtype m == MTAbstract] of
+   Left is -> is
+   Right cycles -> error $ "Cyclic abstract modules: " ++ show cycles
 
 -- | the last abstract in dependency order (head of list)
-greatestAbstract :: Eq i => MGrammar i a -> Maybe i
+greatestAbstract :: (Ord i, Show i) => MGrammar i a -> Maybe i
 greatestAbstract gr = case allAbstracts gr of
   [] -> Nothing
   as -> return $ last as