diff --git a/executables/APITests/MapTests.hs b/executables/APITests/MapTests.hs
--- a/executables/APITests/MapTests.hs
+++ b/executables/APITests/MapTests.hs
@@ -72,6 +72,20 @@
 -- * Tests
 -------------------------
 
+prop_sizeAndList =
+  forAll gen prop
+  where
+    gen = do
+      keys <- nub <$> listOf (arbitrary :: Gen Char)
+      mapM (liftA2 (flip (,)) (arbitrary :: Gen Int) . pure) keys
+    prop list =
+      length list == stmMapLength
+      where
+        stmMapLength =
+          unsafePerformIO $ atomically $ do
+            x <- stmMapFromList list
+            STMMap.size x
+
 prop_fromListToListIsomorphism =
   forAll gen prop
   where
diff --git a/executables/ConcurrentInsertionBench.hs b/executables/ConcurrentInsertionBench.hs
--- a/executables/ConcurrentInsertionBench.hs
+++ b/executables/ConcurrentInsertionBench.hs
@@ -119,12 +119,12 @@
           [
             bgroup "STM Containers"
               [
-                bench "Focus-based" $ 
+                bench "Focus-based" $ nfIO $
                   scSessionRunner focusSCInterpreter threadTransactions,
-                bench "Specialized" $ 
+                bench "Specialized" $ nfIO $
                   scSessionRunner specializedSCInterpreter threadTransactions
               ],
-            bench "Unordered Containers" $
+            bench "Unordered Containers" $ nfIO $
               ucSessionRunner threadTransactions
           ]
   where
diff --git a/executables/ConcurrentTransactionsBench.hs b/executables/ConcurrentTransactionsBench.hs
--- a/executables/ConcurrentTransactionsBench.hs
+++ b/executables/ConcurrentTransactionsBench.hs
@@ -202,7 +202,7 @@
         map concat $! 
         slices (length transactionsGroups `div` threadsNum) transactionsGroups
       in
-        bench (shows threadsNum . showString "/" . shows (transactionsNum `div` threadsNum) $ "") $
+        bench (shows threadsNum . showString "/" . shows (transactionsNum `div` threadsNum) $ "") $ nfIO $
           scSessionRunner specializedSCInterpreter session
   where
     threadsNums = [1, 2, 4, 6, 8, 12, 16, 32, 40, 52, 64, 80, 128]
diff --git a/executables/InsertionBench.hs b/executables/InsertionBench.hs
--- a/executables/InsertionBench.hs
+++ b/executables/InsertionBench.hs
@@ -16,12 +16,12 @@
     [
       bgroup "STM Containers"
       [
-        bench "focus-based" $ 
+        bench "focus-based" $ nfIO $
           do
             t <- atomically $ STMContainers.new :: IO (STMContainers.Map Text.Text ())
             forM_ keys $ \k -> atomically $ STMContainers.focus (Focus.insertM ()) k t
         ,
-        bench "specialized" $
+        bench "specialized" $ nfIO $
           do
             t <- atomically $ STMContainers.new :: IO (STMContainers.Map Text.Text ())
             forM_ keys $ \k -> atomically $ STMContainers.insert () k t
@@ -33,7 +33,7 @@
       bench "Containers" $
         nf (foldr (\k -> Containers.insert k ()) Containers.empty) keys
       ,
-      bench "Hashtables" $ 
+      bench "Hashtables" $ nfIO $
         do
           t <- Hashtables.new :: IO (Hashtables.BasicHashTable Text.Text ())
           forM_ keys $ \k -> Hashtables.insert t k ()
diff --git a/library/STMContainers/Bimap.hs b/library/STMContainers/Bimap.hs
--- a/library/STMContainers/Bimap.hs
+++ b/library/STMContainers/Bimap.hs
@@ -14,6 +14,7 @@
   focus1,
   focus2,
   null,
+  size,
   stream,
 )
 where
@@ -61,6 +62,12 @@
 {-# INLINABLE null #-}
 null :: Bimap a b -> STM Bool
 null = Map.null . m1
+
+-- |
+-- Get the number of elements.
+{-# INLINE size #-}
+size :: Bimap a b -> STM Int
+size = Map.size . m1
 
 -- |
 -- Look up a right value by a left value.
diff --git a/library/STMContainers/HAMT/Nodes.hs b/library/STMContainers/HAMT/Nodes.hs
--- a/library/STMContainers/HAMT/Nodes.hs
+++ b/library/STMContainers/HAMT/Nodes.hs
@@ -159,3 +159,17 @@
     Nodes n -> stream (Level.succ l) n
     Leaf _ e -> return e
     Leaves _ a -> ListT.fromFoldable a
+
+size :: Nodes e -> STM Int
+size nodes =
+  readTVar nodes >>= foldlM step 0
+  where
+    step a =
+      fmap (a+) . nodeSize
+      where
+        nodeSize :: Node e -> STM Int
+        nodeSize =
+          \case
+            Nodes nodes -> size nodes
+            Leaf _ _ -> pure 1
+            Leaves _ x -> pure (SizedArray.size x)
diff --git a/library/STMContainers/Map.hs b/library/STMContainers/Map.hs
--- a/library/STMContainers/Map.hs
+++ b/library/STMContainers/Map.hs
@@ -9,6 +9,7 @@
   lookup,
   focus,
   null,
+  size,
   stream,
 )
 where
@@ -87,6 +88,12 @@
 {-# INLINE null #-}
 null :: Map k v -> STM Bool
 null (Map h) = HAMT.null h
+
+-- |
+-- Get the number of elements.
+{-# INLINE size #-}
+size :: Map k v -> STM Int
+size (Map h) = HAMTNodes.size h
 
 -- |
 -- Stream associations.
diff --git a/library/STMContainers/Multimap.hs b/library/STMContainers/Multimap.hs
--- a/library/STMContainers/Multimap.hs
+++ b/library/STMContainers/Multimap.hs
@@ -10,6 +10,7 @@
   delete,
   deleteByKey,
   lookup,
+  lookupByKey,
   focus,
   null,
   stream,
@@ -26,7 +27,7 @@
 
 -- |
 -- A multimap, based on an STM-specialized hash array mapped trie.
--- 
+--
 -- Basically it's just a wrapper API around @'Map.Map' k ('Set.Set' v)@.
 newtype Multimap k v = Multimap (Map.Map k (Set.Set v))
   deriving (Typeable)
@@ -47,19 +48,25 @@
 -- Look up an item by a value and a key.
 {-# INLINE lookup #-}
 lookup :: (Association k v) => v -> k -> Multimap k v -> STM Bool
-lookup v k (Multimap m) = 
+lookup v k (Multimap m) =
   maybe (return False) (Set.lookup v) =<< Map.lookup k m
 
 -- |
+-- Look up all values by key.
+{-# INLINE lookupByKey #-}
+lookupByKey :: Key k => k -> Multimap k v -> STM (Maybe (Set.Set v))
+lookupByKey k (Multimap m) = Map.lookup k m
+
+-- |
 -- Insert an item.
 {-# INLINABLE insert #-}
 insert :: (Association k v) => v -> k -> Multimap k v -> STM ()
 insert v k (Multimap m) =
   Map.focus ms k m
   where
-    ms = 
-      \case 
-        Just s -> 
+    ms =
+      \case
+        Just s ->
           do
             Set.insert v s
             return ((), Focus.Keep)
@@ -76,9 +83,9 @@
 delete v k (Multimap m) =
   Map.focus ms k m
   where
-    ms = 
-      \case 
-        Just s -> 
+    ms =
+      \case
+        Just s ->
           do
             Set.delete v s
             Set.null s >>= returnDecision . bool Focus.Keep Focus.Remove
@@ -91,23 +98,23 @@
 -- Delete all values associated with a key.
 {-# INLINEABLE deleteByKey #-}
 deleteByKey :: Key k => k -> Multimap k v -> STM ()
-deleteByKey k (Multimap m) = 
+deleteByKey k (Multimap m) =
   Map.delete k m
 
 -- |
 -- Focus on an item with a strategy by a value and a key.
--- 
+--
 -- This function allows to perform simultaneous lookup and modification.
--- 
+--
 -- The strategy is over a unit since we already know,
 -- which value we're focusing on and it doesn't make sense to replace it,
 -- however we still can decide wether to keep or remove it.
 {-# INLINE focus #-}
 focus :: (Association k v) => Focus.StrategyM STM () r -> v -> k -> Multimap k v -> STM r
-focus = 
+focus =
   \s v k (Multimap m) -> Map.focus (liftSetItemStrategy v s) k m
   where
-    liftSetItemStrategy :: 
+    liftSetItemStrategy ::
       (Set.Element e) => e -> Focus.StrategyM STM () r -> Focus.StrategyM STM (Set.Set e) r
     liftSetItemStrategy e s =
       \case
@@ -123,7 +130,7 @@
                     return (Focus.Replace s)
                 _ ->
                   return Focus.Keep
-        Just set -> 
+        Just set ->
           do
             r <- Set.focus s e set
             (r,) . bool Focus.Keep Focus.Remove <$> Set.null set
@@ -136,8 +143,8 @@
 
 -- |
 -- Construct a new multimap in IO.
--- 
--- This is useful for creating it on a top-level using 'unsafePerformIO', 
+--
+-- This is useful for creating it on a top-level using 'unsafePerformIO',
 -- because using 'atomically' inside 'unsafePerformIO' isn't possible.
 {-# INLINE newIO #-}
 newIO :: IO (Multimap k v)
@@ -151,11 +158,11 @@
 
 -- |
 -- Stream associations.
--- 
--- Amongst other features this function provides an interface to folding 
+--
+-- Amongst other features this function provides an interface to folding
 -- via the 'ListT.fold' function.
 stream :: Multimap k v -> ListT STM (k, v)
-stream (Multimap m) = 
+stream (Multimap m) =
   Map.stream m >>= \(k, s) -> (k,) <$> Set.stream s
 
 -- |
diff --git a/library/STMContainers/Prelude.hs b/library/STMContainers/Prelude.hs
--- a/library/STMContainers/Prelude.hs
+++ b/library/STMContainers/Prelude.hs
@@ -1,8 +1,6 @@
 module STMContainers.Prelude
 ( 
   module Exports,
-  bug,
-  bottom,
   traversePair,
 )
 where
@@ -11,10 +9,6 @@
 -------------------------
 import BasePrelude as Exports
 
--- placeholders
--------------------------
-import Development.Placeholders as Exports
-
 -- hashable
 -------------------------
 import Data.Hashable as Exports (Hashable(..))
@@ -26,16 +20,6 @@
 -- list-t
 -------------------------
 import ListT as Exports (ListT)
-
--- custom
--------------------------
-import qualified Debug.Trace.LocationTH
-
-bug = [e| $(Debug.Trace.LocationTH.failure) . (msg <>) |]
-  where
-    msg = "A \"stm-containers\" package bug: " :: String
-
-bottom = [e| $bug "Bottom evaluated" |]
 
 -- | A replacement for the missing 'Traverse' instance of pair in base < 4.7.
 traversePair :: Functor f => (a -> f b) -> (c, a) -> f (c, b)
diff --git a/library/STMContainers/Set.hs b/library/STMContainers/Set.hs
--- a/library/STMContainers/Set.hs
+++ b/library/STMContainers/Set.hs
@@ -9,6 +9,7 @@
   lookup,
   focus,
   null,
+  size,
   stream,
 )
 where
@@ -91,6 +92,12 @@
 {-# INLINE null #-}
 null :: Set e -> STM Bool
 null = HAMT.null . hamt
+
+-- |
+-- Get the number of elements.
+{-# INLINE size #-}
+size :: Set e -> STM Int
+size (Set h) = HAMTNodes.size h
 
 -- |
 -- Stream elements.
diff --git a/stm-containers.cabal b/stm-containers.cabal
--- a/stm-containers.cabal
+++ b/stm-containers.cabal
@@ -1,7 +1,7 @@
 name:
   stm-containers
 version:
-  0.2.10
+  0.2.11
 synopsis:
   Containers for STM
 description:
@@ -66,9 +66,6 @@
     list-t >= 0.2 && < 0.5,
     focus >= 0.1.2 && < 0.2,
     transformers >= 0.2 && < 0.6,
-    -- debugging:
-    loch-th == 0.2.*,
-    placeholders == 0.1.*,
     -- general:
     primitive >= 0.5 && < 0.7,
     base-prelude < 2