diff --git a/Data/Histogram.hs b/Data/Histogram.hs
--- a/Data/Histogram.hs
+++ b/Data/Histogram.hs
@@ -35,6 +35,8 @@
   , HistIndex(..) 
   , histIndex
   , at
+  , atV
+  , atI
     -- * Transformations
   , map
   , bmap
@@ -130,6 +132,14 @@
 
 at :: (Bin bin, Unbox a) => Histogram bin a -> HistIndex bin -> a
 at = H.at
+
+atV :: (Bin bin, Unbox a) => Histogram bin a -> BinValue bin -> a
+atV = H.atV
+
+atI :: (Bin bin, Unbox a) => Histogram bin a -> Int -> a
+atI = H.atI
+
+
 
 ----------------------------------------------------------------
 -- Modify histograms
diff --git a/Data/Histogram/Fill.hs b/Data/Histogram/Fill.hs
--- a/Data/Histogram/Fill.hs
+++ b/Data/Histogram/Fill.hs
@@ -26,7 +26,7 @@
   , joinHBuilderM
   , treeHBuilderM
     -- ** Stateless
-  , HBuilder
+  , HBuilder(HBuilder)
   , toHBuilderST
   , toHBuilderIO
   , joinHBuilder
@@ -45,6 +45,8 @@
   , mkFoldBuilderG
     -- ** Pure fold
   , mkFolder
+    -- ** Generic constructors
+  , mkStatefulBuilder
     -- * Fill histograms
   , fillBuilder
   , fillBuilderVec
@@ -74,6 +76,8 @@
 import Data.Histogram.Bin
 import Data.Histogram.ST
 
+
+
 ----------------------------------------------------------------
 -- Type class
 ----------------------------------------------------------------
@@ -194,16 +198,18 @@
 -- Monadic builder
 ----------------------------------------------------------------
 
--- | Stateful histogram builder. There is no direct way to construct
---   such builder. Only way to do it is to create 'HBuilder' and use
---   'toHBuilderST' or 'toHBuilderIO'.
+-- | Stateful histogram builder. Adding value to builder could be done
+--   with 'feedOne' and result could be extracted with
+--   'freezeHBuilderM'.
 --
---   It's useful when result should be extracted many times from the
---   same accumulator.
+--   There are two ways to obtain stateful builder. First and
+--   recommended is to thaw 'HBuilder' using 'toHBuilderIO' or
+--   'toHBuilderST'. Second is to use 'mkStatefulBuilder'.
 data HBuilderM m a b = HBuilderM { hbInput  :: a -> m ()
                                  , hbOutput :: m b
                                  }
 
+-- | Builders modified using 'HistBuilder' API will share same buffer.
 instance PrimMonad m => HistBuilder (HBuilderM m) where
     modifyIn    f h = h { hbInput  = hbInput h . f }
     addCut      f h = h { hbInput  = \x -> when (f x) (hbInput h x) }
@@ -236,8 +242,8 @@
 feedOne = hbInput
 {-# INLINE feedOne #-}
 
--- | Create stateful histogram from instructions. Histograms could
---   be filled either in the ST monad or with createHistograms
+-- | Extract result from histogram builder. It's safe to call this
+--   function multiple times and mutate builder afterwards.
 freezeHBuilderM :: PrimMonad m => HBuilderM m a b -> m b
 freezeHBuilderM = hbOutput
 {-# INLINE freezeHBuilderM #-}
@@ -254,23 +260,29 @@
 treeHBuilderM fs h = joinHBuilderM $ fmap ($ h) fs
 {-# INLINE treeHBuilderM #-}
 
+
+
 ----------------------------------------------------------------
 -- Stateless
 ----------------------------------------------------------------
 
--- | Stateless histogram builder
-newtype HBuilder a b = HBuilder { toHBuilderST :: forall s . ST s (HBuilderM (ST s) a b)
-                                  -- ^ Convert builder to stateful builder in ST monad
-                                }
+-- | Wrapper around stateful histogram builder. It is much more
+--   convenient to work with than 'HBuilderM'.
+newtype HBuilder a b = HBuilder (forall s . ST s (HBuilderM (ST s) a b))
 
+-- | Convert builder to stateful builder in ST monad
+toHBuilderST :: HBuilder a b -> ST s (HBuilderM (ST s) a b)
+{-# INLINE toHBuilderST #-}
+toHBuilderST (HBuilder hb) = hb
+
 -- | Convert builder to builder in IO monad
 toHBuilderIO :: HBuilder a b -> IO (HBuilderM IO a b)
+{-# INLINE toHBuilderIO #-}
 toHBuilderIO (HBuilder h) = do
   builder <- stToIO h
   return (HBuilderM
           (stToIO . hbInput builder)
           (stToIO $ hbOutput builder))
-{-# INLINE toHBuilderIO #-}
 
 instance HistBuilder (HBuilder) where
     modifyIn    f (HBuilder h) = HBuilder (modifyIn  f <$> h)
@@ -288,6 +300,7 @@
     mappend h g = mappend <$> h <*> g
     mconcat     = fmap mconcat . joinHBuilder
     {-# INLINE mempty  #-}
+    {-# INLINE mappend #-}
     {-# INLINE mconcat #-}
 
 -- | Join hitogram builders in container.
@@ -378,9 +391,10 @@
 
 
 -- | Create histogram builder which just does ordinary pure fold. It
--- is intended for use when some fold should be performed together
--- with histogram filling
+--   is intended for use when some fold should be performed together
+--   with histogram filling
 mkFolder :: b -> (a -> b -> b) -> HBuilder a b
+{-# INLINE mkFolder #-}
 mkFolder a f = HBuilder $ do
   ref <- newSTRef a
   return HBuilderM { hbInput  = \x -> do acc <- readSTRef ref
@@ -388,7 +402,17 @@
                                          writeSTRef ref acc'
                    , hbOutput = readSTRef ref
                    }
-{-# INLINE mkFolder #-}
+
+
+-- | Create stateful histogram builder. Output function should be safe
+--   to call multiple times and builder could be modified afterwards.
+--   So functions like @unsafeFreeze@ from @vector@ couldn't be used.
+mkStatefulBuilder :: PrimMonad m
+                  => (a -> m ()) -- ^ Add value to accumulator
+                  -> m b         -- ^ Extract result from accumulator
+                  -> HBuilderM m a b
+{-# INLINE mkStatefulBuilder #-}
+mkStatefulBuilder = HBuilderM
 
 
 
diff --git a/Data/Histogram/Generic.hs b/Data/Histogram/Generic.hs
--- a/Data/Histogram/Generic.hs
+++ b/Data/Histogram/Generic.hs
@@ -33,6 +33,8 @@
   , HistIndex(..) 
   , histIndex
   , at
+  , atV
+  , atI
     -- * Transformations
   , map
   , bmap
@@ -269,6 +271,16 @@
 -- | Index histogtam.
 at :: (Bin bin, Vector v a) => Histogram v bin a -> HistIndex bin -> a
 at (Histogram bin _ v) i = v ! histIndex bin i
+
+-- | Index histogram using bin value
+atV :: (Bin bin, Vector v a) => Histogram v bin a -> BinValue bin -> a
+atV h = at h . Value
+
+-- | Index histogram using vector index
+atI :: (Bin bin, Vector v a) => Histogram v bin a -> Int -> a
+atI h = at h . Index
+
+
 
 ----------------------------------------------------------------
 -- Transformation
diff --git a/histogram-fill.cabal b/histogram-fill.cabal
--- a/histogram-fill.cabal
+++ b/histogram-fill.cabal
@@ -1,9 +1,15 @@
 Name:           histogram-fill
-Version:        0.7.2.0
+Version:        0.7.3.0
 Synopsis:       Library for histograms creation.
 Description:    
   This is library for histograms filling. Its aim to provide
   convenient way to create and fill histograms. 
+  .
+  Changes in 0.7.3.0
+  .
+  * @mkStatefulBuilder@ is added and HBuilder constructor is exposed.
+  .
+  * Indexing operators for immutable histograms are added.
   .
   Changes in 0.7.2.0
   .