diff --git a/Data/PoliMorf.hs b/Data/PoliMorf.hs
--- a/Data/PoliMorf.hs
+++ b/Data/PoliMorf.hs
@@ -26,7 +26,8 @@
 ) where
 
 import Control.Applicative ((<$>), (<*>))
-import Data.Monoid (Monoid, mappend)
+import Data.Maybe (catMaybes)
+import Data.Monoid (mappend)
 import Data.List (foldl')
 import Data.Binary (Binary, get, put)
 import qualified Data.Map as M
@@ -44,11 +45,15 @@
 -- | A morphosyntactic tag.
 type Tag  = T.Text
 
+-- | A category.
+type Cat  = T.Text
+
 -- | An entry from the PoliMorf dictionary.
 data Entry = Entry
     { form :: !Form
     , base :: !Base
-    , tag  :: !Tag }
+    , tag  :: !Tag
+    , cat  :: !Cat }
     deriving (Eq, Ord, Show, Read)
 
 -- | Read the PoliMorf from the file.
@@ -62,7 +67,7 @@
 -- | Get an entry pair from a PoliMorf row.
 parsePoliRow :: L.Text -> Entry
 parsePoliRow row = case map L.toStrict (L.split (=='\t') row) of
-    [_form, _base, _tag] -> Entry _form _base _tag
+    [_form, _base, _tag, _cat] -> Entry _form _base _tag _cat
     _   -> error $ "parsePoliRow: invalid row \"" ++ L.unpack row ++ "\""
 
 -- | A map from forms to their possible base forms (there may be many since
@@ -92,34 +97,36 @@
         c   -> error $ "get: invalid RelCode code '" ++ [c] ++ "'"
 
 -- | Merge the 'BaseMap' with the dictionary resource which maps forms to
--- monoidal labels.  Depending on the inference technique there are three
--- kinds of labels in the resultant dictionary:
+-- sets of labels.  Every label is assigned a 'RelCode' which tells what
+-- is the relation between the label and the form.  There are three
+-- kinds of labels:
 -- 'Exact' labels assigned in a direct manner, 'ByBase' labels assigned
 -- to all forms which have a base form with a label in the input dictionary,
 -- and 'ByForm' labels assigned to all forms which have a related form from the
 -- same lexeme with a label in the input dictionary.
 --
--- For a particular form in the output dictionary there are labels extracted
--- with at most one of the methods described above, with 'Exact' labels
--- having a precedence over 'ByBase' labels and 'ByBase' labels having
--- a precedence over 'ByForm' labels.
---
 -- This function is far from being memory efficient right now.  If you plan to
 -- run it with respect to the entire PoliMorf dictionary you should do it
 -- on a machine with an abundance of available memory.
-merge :: Monoid m => BaseMap -> M.Map Form m -> M.Map Form (Maybe (m, RelCode))
-merge poli dict0 =
-    M.fromList [(x, combine x) | x <- keys]
+merge
+    :: Ord a => BaseMap
+    -> M.Map Form (S.Set a)
+    -> M.Map Form (M.Map a RelCode)
+merge poli dict0 = M.fromList
+    [ (x, combine x)
+    | x <- keys ]
   where
     -- Keys in the output dictionary.
     keys = S.toList (M.keysSet poli `S.union` M.keysSet dict0)
 
     -- Combining function.
-    combine x
-        | Just y <- M.lookup x dict0 = Just (y, Exact)
-        | Just y <- M.lookup x dict1 = Just (y, ByBase)
-        | Just y <- M.lookup x dict2 = Just (y, ByForm)
-        | otherwise = Nothing
+    combine x = (M.unionsWith min . catMaybes)
+        [ label Exact  <$> M.lookup x dict0 
+        , label ByBase <$> M.lookup x dict1
+        , label ByForm <$> M.lookup x dict2 ]
+
+    label :: Ord a => RelCode -> S.Set a -> M.Map a RelCode
+    label code s = M.fromList [(x, code) | x <- S.toList s]
 
     -- Extended to all base forms of dict0 keys.
     dict1 = fromListWith mappend
diff --git a/polimorf.cabal b/polimorf.cabal
--- a/polimorf.cabal
+++ b/polimorf.cabal
@@ -1,5 +1,5 @@
 name:               polimorf
-version:            0.3.1
+version:            0.4.0
 synopsis:           Working with the PoliMorf dictionary
 description:
     The library provides functionality for manipulating PoliMorf, the
@@ -31,4 +31,4 @@
 
 source-repository head
     type: git
-    location: git://github.com/kawu/polimorf.git
+    location: https://github.com/kawu/polimorf.git