diff --git a/library/PrimitiveExtras/Bitmap.hs b/library/PrimitiveExtras/Bitmap.hs
--- a/library/PrimitiveExtras/Bitmap.hs
+++ b/library/PrimitiveExtras/Bitmap.hs
@@ -1,6 +1,6 @@
 module PrimitiveExtras.Bitmap
 (
-  Bitmap,
+  Bitmap(..),
   empty,
   singleton,
   insert,
diff --git a/library/PrimitiveExtras/By6Bits.hs b/library/PrimitiveExtras/By6Bits.hs
--- a/library/PrimitiveExtras/By6Bits.hs
+++ b/library/PrimitiveExtras/By6Bits.hs
@@ -7,7 +7,7 @@
   pair,
   insert,
   replace,
-  update,
+  adjust,
   unset,
   lookup,
   focusAt,
@@ -90,29 +90,31 @@
     sparseIndex = Bitmap.populatedIndex i b
     in By6Bits b (SmallArray.set sparseIndex e a)
 
-{-# INLINE update #-}
-update :: (e -> e) -> Int -> By6Bits e -> By6Bits e
-update fn i (By6Bits b a) =
+{-# INLINE adjust #-}
+adjust :: (e -> e) -> Int -> By6Bits e -> By6Bits e
+adjust fn i (By6Bits b a) =
   let
     sparseIndex = Bitmap.populatedIndex i b
     in
       By6Bits b
-        (SmallArray.unsafeUpdate fn sparseIndex a)
+        (SmallArray.unsafeAdjust fn sparseIndex a)
 
 -- |
 -- Remove an element.
 {-# INLINE unset #-}
 unset :: Int -> By6Bits e -> By6Bits e
-unset i (By6Bits b a) =
+unset i (By6Bits (Bitmap b) a) =
   {-# SCC "unset" #-}
-  if Bitmap.isPopulated i b
-    then
-      let
-        sparseIndex = Bitmap.populatedIndex i b
-        b' = Bitmap.invert i b
-        a' = SmallArray.unset sparseIndex a
-        in By6Bits b' a'
-    else By6Bits b a
+  let
+    bitAtIndex = bit i
+    isPopulated = b .&. bitAtIndex /= 0
+    in if isPopulated
+      then let
+        populatedIndex = popCount (b .&. pred bitAtIndex)
+        updatedBitmap = xor b bitAtIndex
+        updatedArray = SmallArray.unset populatedIndex a
+        in By6Bits (Bitmap updatedBitmap) updatedArray
+      else By6Bits (Bitmap b) a
 
 -- |
 -- Lookup an item at the index.
diff --git a/library/PrimitiveExtras/SmallArray.hs b/library/PrimitiveExtras/SmallArray.hs
--- a/library/PrimitiveExtras/SmallArray.hs
+++ b/library/PrimitiveExtras/SmallArray.hs
@@ -48,12 +48,11 @@
 set index a array =
   {-# SCC "set" #-} 
   let
-    !size = sizeofSmallArray array
-    in runSmallArray $ do
-      newMa <- newSmallArray size undefined
-      copySmallArray newMa 0 array 0 size
-      writeSmallArray newMa index a
-      return newMa
+    size = sizeofSmallArray array
+    in runST $ do
+      m <- thawSmallArray array 0 size
+      writeSmallArray m index a
+      unsafeFreezeSmallArray m
 
 {-# INLINE insert #-}
 insert :: Int -> a -> SmallArray a -> SmallArray a
@@ -70,23 +69,23 @@
       copySmallArray newMa nextIndex array index amountOfFollowingElements
       return newMa
 
-{-# INLINE update #-}
-update :: (a -> a) -> Int -> SmallArray a -> SmallArray a
-update fn index array =
+{-# INLINE adjust #-}
+adjust :: (a -> a) -> Int -> SmallArray a -> SmallArray a
+adjust fn index array =
   let
     size = sizeofSmallArray array
     in if size > index && index >= 0
-      then unsafeUpdateWithSize fn index size array
+      then unsafeAdjustWithSize fn index size array
       else array
 
-{-# INLINE unsafeUpdate #-}
-unsafeUpdate :: (a -> a) -> Int -> SmallArray a -> SmallArray a
-unsafeUpdate fn index array =
-  unsafeUpdateWithSize fn index (sizeofSmallArray array) array
+{-# INLINE unsafeAdjust #-}
+unsafeAdjust :: (a -> a) -> Int -> SmallArray a -> SmallArray a
+unsafeAdjust fn index array =
+  unsafeAdjustWithSize fn index (sizeofSmallArray array) array
 
-{-# INLINE unsafeUpdateWithSize #-}
-unsafeUpdateWithSize :: (a -> a) -> Int -> Int -> SmallArray a -> SmallArray a
-unsafeUpdateWithSize fn index size array =
+{-# INLINE unsafeAdjustWithSize #-}
+unsafeAdjustWithSize :: (a -> a) -> Int -> Int -> SmallArray a -> SmallArray a
+unsafeAdjustWithSize fn index size array =
   runST $ do
     m <- thawSmallArray array 0 size
     element <- readSmallArray m index
@@ -121,6 +120,19 @@
     | otherwise -> do
       a <- newSmallArray 1 e2
       return a
+
+{-# INLINE findAndReplace #-}
+findAndReplace :: (a -> Maybe a) -> SmallArray a -> SmallArray a
+findAndReplace f array =
+  let
+    size = sizeofSmallArray array
+    iterate index =
+      if index < size
+        then case f (indexSmallArray array index) of
+          Just newElement -> set index newElement array
+          Nothing -> iterate (succ index)
+        else array
+    in iterate 0
 
 {-# INLINE find #-}
 find :: (a -> Bool) -> SmallArray a -> Maybe a
diff --git a/primitive-extras.cabal b/primitive-extras.cabal
--- a/primitive-extras.cabal
+++ b/primitive-extras.cabal
@@ -1,5 +1,5 @@
 name: primitive-extras
-version: 0.9
+version: 0.10
 category: Primitive
 synopsis: Extras for the "primitive" library
 homepage: https://github.com/metrix-ai/primitive-extras