diff --git a/changeset-lens.cabal b/changeset-lens.cabal
--- a/changeset-lens.cabal
+++ b/changeset-lens.cabal
@@ -1,6 +1,6 @@
 cabal-version: 2.4
 name: changeset-lens
-version: 0.1.1
+version: 0.2
 synopsis: Stateful monad transformer based on monoidal actions
 description:
   A general state monad transformer with separate types for the state and the possible changes.
@@ -64,8 +64,8 @@
     Control.Monad.Changeset.Lens.Setter
 
   build-depends:
-    base >=4.12 && <4.22,
-    changeset ==0.1.1,
+    base >=4.16 && <4.23,
+    changeset ==0.2,
     containers >=0.6 && <0.8,
     indexed-traversable ^>=0.1.1,
     lens >=4.19.2 && <5.4,
diff --git a/src/Control/Monad/Changeset/Lens.hs b/src/Control/Monad/Changeset/Lens.hs
--- a/src/Control/Monad/Changeset/Lens.hs
+++ b/src/Control/Monad/Changeset/Lens.hs
@@ -11,8 +11,11 @@
 import Prelude hiding (Foldable (..))
 
 -- lens
-import Control.Lens (Lens', Prism', Traversal')
+import Control.Lens (Fold, Iso', Lens', Prism', Setter', Traversal', iso, over, to)
 
+-- changeset
+import Data.Monoid.RightAction (RightAction (..), RightTorsor (..))
+
 -- changeset-lens
 import Control.Monad.Changeset.Lens.Setter
 
@@ -27,3 +30,23 @@
 -- | Create a changeset that changes those parts of a state which are traversed
 traversalChangeset :: Traversal' s a -> w -> SetterChangeset s a w
 traversalChangeset t w = setterChangeset t w
+
+-- | Given a setter, change the part of the state it focusses.
+changing :: (RightAction w a) => Setter' s a -> s -> w -> s
+changing setter s w = over setter (`actRight` w) s
+
+-- | Apply a change to each focussed value.
+changed :: (RightAction w s) => w -> Fold s s
+changed w = to (`actRight` w)
+
+{- | Witness the isomorphism between state and torsor change.
+
+A torsor change is a similar type to the state, but without a base point.
+Given such a base point, a state value is converted to the change by 'differenceRight',
+and a change is converted to a state value by 'actRight'.
+
+In other words, when you fix a base state value,
+this isomorphism will translate to and from the change necessary to apply to the base state to get to the given state.
+-}
+relativeTo :: (RightTorsor w s) => s -> Iso' s w
+relativeTo sReference = iso (differenceRight sReference) (actRight sReference)