diff --git a/.travis.yml b/.travis.yml
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 # This Travis job script has been generated by a script via
 #
-#   make_travis_yml_2.hs 'lens.cabal'
+#   runghc make_travis_yml_2.hs '-o' '.travis.yml' '--irc-channel=irc.freenode.org#haskell-lens' '--no-no-tests-no-bench' '--no-installed' 'cabal.project'
 #
 # For more information, see https://github.com/hvr/multi-ghc-travis
 #
@@ -10,6 +10,14 @@
 git:
   submodules: false  # whether to recursively clone submodules
 
+notifications:
+  irc:
+    channels:
+      - "irc.freenode.org#haskell-lens"
+    skip_join: true
+    template:
+      - "\x0313lens\x03/\x0306%{branch}\x03 \x0314%{commit}\x03 %{build_url} %{message}"
+
 cache:
   directories:
     - $HOME/.cabal/packages
@@ -24,70 +32,134 @@
   - rm -fv $HOME/.cabal/packages/hackage.haskell.org/01-index.tar
   - rm -fv $HOME/.cabal/packages/hackage.haskell.org/01-index.tar.idx
 
+  - rm -rfv $HOME/.cabal/packages/head.hackage
+
 addons:
   apt:
     packages: &apt_packages
       - ghc-ppa-tools
+      - freeglut3-dev
       - alex-3.1.7
       - happy-1.19.5
 
 matrix:
   include:
-    - compiler: "ghc-8.2.1"
-    # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-8.2.1], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.0.2"
+    - compiler: "ghc-7.4.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-8.0.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.10.3"
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.4.2], sources: [hvr-ghc]}}
+    - compiler: "ghc-7.6.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.10.3], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.6.3], sources: [hvr-ghc]}}
     - compiler: "ghc-7.8.4"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
       addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.8.4], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.6.3"
+    - compiler: "ghc-7.10.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.6.3], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.4.2"
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.10.3], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.0.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.4.2], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-8.0.2], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.2.2"
+    # env: TEST=--disable-tests BENCH=--disable-benchmarks
+      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-8.2.2], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.4.1"
+      env: GHCHEAD=true
+      addons: {apt: {packages: [*apt_packages,cabal-install-head,ghc-8.4.1], sources: [hvr-ghc]}}
+    - compiler: "ghc-head"
+      env: GHCHEAD=true
+      addons: {apt: {packages: [*apt_packages,cabal-install-head,ghc-head], sources: [hvr-ghc]}}
 
+  allow_failures:
+    - compiler: "ghc-8.4.1"
+    - compiler: "ghc-head"
+
 before_install:
- - HC=${CC}
- - unset CC
- - export HAPPYVER=1.19.5
- - export ALEXVER=3.1.7
- - export HLINTVER=2.0.9
- - mkdir ~/.hlint
- - curl -L https://github.com/ndmitchell/hlint/releases/download/v$HLINTVER/hlint-$HLINTVER-x86_64-linux.tar.gz | tar -xz --strip-components=1 -C ~/.hlint
- - PATH=/opt/ghc/bin:/opt/ghc-ppa-tools/bin:~/.hlint:~/.cabal/bin:/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:/opt/happy/$HAPPYVER/bin:/opt/alex/$ALEXVER/bin:$PATH
+  - HC=${CC}
+  - HCPKG=${HC/ghc/ghc-pkg}
+  - unset CC
+  - export HAPPYVER=1.19.5
+  - export ALEXVER=3.1.7
+  - export HLINTVER=2.0.9
+  - mkdir ~/.hlint
+  - curl -L https://github.com/ndmitchell/hlint/releases/download/v$HLINTVER/hlint-$HLINTVER-x86_64-linux.tar.gz | tar -xz --strip-components=1 -C ~/.hlint
+  - ROOTDIR=$(pwd)
+  - mkdir -p $HOME/.local/bin
+  - "PATH=~/.hlint:/opt/ghc/bin:/opt/ghc-ppa-tools/bin:$HOME/local/bin:/opt/happy/$HAPPYVER/bin:/opt/alex/$ALEXVER/bin:$PATH"
+  - HCNUMVER=$(( $(${HC} --numeric-version|sed -E 's/([0-9]+)\.([0-9]+)\.([0-9]+).*/\1 * 10000 + \2 * 100 + \3/') ))
+  - echo $HCNUMVER
 
 install:
- - cabal --version
- - echo "$(${HC} --version) [$(${HC} --print-project-git-commit-id 2> /dev/null || echo '?')]"
- - BENCH=${BENCH---enable-benchmarks}
- - TEST=${TEST---enable-tests}
- - travis_retry cabal update -v
- - sed -i 's/^jobs:/-- jobs:/' ${HOME}/.cabal/config
- - rm -fv cabal.project.local
- - rm -f cabal.project.freeze
- - cabal new-build -w ${HC} ${TEST} ${BENCH} --dep -j
+  - cabal --version
+  - echo "$(${HC} --version) [$(${HC} --print-project-git-commit-id 2> /dev/null || echo '?')]"
+  - BENCH=${BENCH---enable-benchmarks}
+  - TEST=${TEST---enable-tests}
+  - HADDOCK=${HADDOCK-true}
+  - INSTALLED=${INSTALLED-true}
+  - GHCHEAD=${GHCHEAD-false}
+  - travis_retry cabal update -v
+  - "sed -i.bak 's/^jobs:/-- jobs:/' ${HOME}/.cabal/config"
+  - rm -fv cabal.project cabal.project.local
+  # Overlay Hackage Package Index for GHC HEAD: https://github.com/hvr/head.hackage
+  - |
+    if $GHCHEAD; then
+      sed -i.bak 's/-- allow-newer:.*/allow-newer: *:base, *:template-haskell, *:ghc, *:Cabal/' ${HOME}/.cabal/config
 
+      echo 'repository head.hackage'                                                        >> ${HOME}/.cabal/config
+      echo '   url: http://head.hackage.haskell.org/'                                       >> ${HOME}/.cabal/config
+      echo '   secure: True'                                                                >> ${HOME}/.cabal/config
+      echo '   root-keys: 07c59cb65787dedfaef5bd5f987ceb5f7e5ebf88b904bbd4c5cbdeb2ff71b740' >> ${HOME}/.cabal/config
+      echo '              2e8555dde16ebd8df076f1a8ef13b8f14c66bad8eafefd7d9e37d0ed711821fb' >> ${HOME}/.cabal/config
+      echo '              8f79fd2389ab2967354407ec852cbe73f2e8635793ac446d09461ffb99527f6e' >> ${HOME}/.cabal/config
+      echo '   key-threshold: 3'                                                            >> ${HOME}/.cabal.config
+
+      cabal new-update head.hackage -v
+    fi
+  - grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
+  - "printf 'packages: \".\" \"./examples\" \"./lens-properties\"\\n' > cabal.project"
+  - cat cabal.project
+  - if [ -f "./configure.ac" ]; then
+      (cd "." && autoreconf -i);
+    fi
+  - if [ -f "./examples/configure.ac" ]; then
+      (cd "./examples" && autoreconf -i);
+    fi
+  - if [ -f "./lens-properties/configure.ac" ]; then
+      (cd "./lens-properties" && autoreconf -i);
+    fi
+  - rm -f cabal.project.freeze
+  - cabal new-build -w ${HC} ${TEST} ${BENCH} --project-file="cabal.project" --dep -j2 all
+  - rm -rf "."/.ghc.environment.* "./examples"/.ghc.environment.* "./lens-properties"/.ghc.environment.* "."/dist "./examples"/dist "./lens-properties"/dist
+  - DISTDIR=$(mktemp -d /tmp/dist-test.XXXX)
+
 # Here starts the actual work to be performed for the package under test;
 # any command which exits with a non-zero exit code causes the build to fail.
 script:
- - if [ -f configure.ac ]; then autoreconf -i; fi
- - rm -rf .ghc.environment.* dist/
- # build tests and benchmarks, run tests
- - cabal new-build -w ${HC} ${TEST} ${BENCH}
- - if [ "x$TEST" = "x--enable-tests" ]; then cabal new-test -w ${HC} ${TEST} ${BENCH}; fi
- - hlint src --cpp-define=HLINT --cpp-ansi
+  # test that source-distributions can be generated
+  - (cd "." && cabal sdist)
+  - (cd "./examples" && cabal sdist)
+  - (cd "./lens-properties" && cabal sdist)
+  - mv "."/dist/lens-*.tar.gz "./examples"/dist/lens-examples-*.tar.gz "./lens-properties"/dist/lens-properties-*.tar.gz ${DISTDIR}/
+  - cd ${DISTDIR} || false
+  - find . -maxdepth 1 -name '*.tar.gz' -exec tar -xvf '{}' \;
+  - "printf 'packages: lens-*/*.cabal lens-examples-*/*.cabal lens-properties-*/*.cabal\\n' > cabal.project"
+  - cat cabal.project
 
-notifications:
-  irc:
-    channels:
-      - "irc.freenode.org#haskell-lens"
-    skip_join: true
-    template:
-      - "\x0313lens\x0f/\x0306%{branch}\x0f \x0314%{commit}\x0f %{message} \x0302\x1f%{build_url}\x0f"
+
+  # build & run tests, build benchmarks
+  - cabal new-build -w ${HC} ${TEST} ${BENCH} all
+  - if [ "x$TEST" = "x--enable-tests" ]; then cabal new-test -w ${HC} ${TEST} ${BENCH} all; fi
+
+  # cabal check
+  - (cd lens-* && cabal check)
+  - (cd lens-examples-* && cabal check)
+  - (cd lens-properties-* && cabal check)
+
+  # haddock
+  - rm -rf ./dist-newstyle
+  - if $HADDOCK; then cabal new-haddock -w ${HC} ${TEST} ${BENCH} all; else echo "Skipping haddock generation";fi
+
+  # hlint
+  - (cd lens-* && hlint src --cpp-define=HLINT --cpp-ansi)
+
+# REGENDATA ["-o",".travis.yml","--irc-channel=irc.freenode.org#haskell-lens","--no-no-tests-no-bench","--no-installed","cabal.project"]
 # EOF
diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -1,3 +1,38 @@
+4.16 [2018.01.28]
+-----------------
+* The `Semigroup` instances for `Traversed` and `Sequenced` are now more
+  constrained (going from `Apply` to `Applicative` and `Monad`, respectively).
+  In GHC 8.4, `Semigroup` is a superclass of `Monoid`, therefore we'd need to
+  have `Apply` constraint in the `Monoid` instances. We opted to weaken our
+  ability to use `Apply` than to lose compatibility with third-party packages
+  that don't supply instances for `Apply`.
+
+  In practice this changes the (specialised) type signature of `traverseOf_`
+  ```diff+
+  - traverseOf_ :: Apply f       => Fold1 s a -> (a -> f r) -> s -> f ()
+  + traverseOf_ :: Applicative f => Fold1 s a -> (a -> f r) -> s -> f ()
+  ```
+  and similarly for `forOf_` and `sequenceOf_`.
+
+  As part of this change, new combinators `traverse1Of_`, `for1Of_` and
+  `sequence1Of_` were added for `Apply`-only effects.
+
+  Similar instance context changes were made for `Folding` and `Effect`,
+  but these changes aren't publicly visible.
+
+* Add `Control.Lens.Unsound`, which exports unsound functionality for forming
+  products of lenses and sums of prisms.
+
+* Add `Numeric.Natural.Lens`, which export convenient isomorphisms for
+  natural numbers.
+
+* Add `Strict` instances for strict and lazy `ST`.
+
+* Adapt `Language.Haskell.TH.Lens` for `template-haskell-2.13` (bundled
+  with GHC 8.4).
+
+* Add `Semigroup` and `Monoid` instances for `Indexing`.
+
 4.15.4
 ----
 * `makeFields` and `declareFields` are now smarter with respect to type
diff --git a/README.markdown b/README.markdown
--- a/README.markdown
+++ b/README.markdown
@@ -1,7 +1,7 @@
 Lens: Lenses, Folds, and Traversals
 ==================================
 
-[![Hackage](https://img.shields.io/hackage/v/lens.svg)](https://hackage.haskell.org/package/lens) [![Build Status](https://secure.travis-ci.org/ekmett/lens.svg)](http://travis-ci.org/ekmett/lens)
+[![Hackage](https://img.shields.io/hackage/v/lens.svg)](https://hackage.haskell.org/package/lens) [![Build Status](https://secure.travis-ci.org/ekmett/lens.svg)](http://travis-ci.org/ekmett/lens) [![Hackage Deps](https://img.shields.io/hackage-deps/v/lens.svg)](http://packdeps.haskellers.com/reverse/lens)
 
 This package provides families of [lenses](https://github.com/ekmett/lens/blob/master/src/Control/Lens/Type.hs), [isomorphisms](https://github.com/ekmett/lens/blob/master/src/Control/Lens/Iso.hs), [folds](https://github.com/ekmett/lens/blob/master/src/Control/Lens/Fold.hs), [traversals](https://github.com/ekmett/lens/blob/master/src/Control/Lens/Traversal.hs), [getters](https://github.com/ekmett/lens/blob/master/src/Control/Lens/Getter.hs) and [setters](https://github.com/ekmett/lens/blob/master/src/Control/Lens/Setter.hs).
 
diff --git a/cabal.project b/cabal.project
--- a/cabal.project
+++ b/cabal.project
@@ -1,1 +1,3 @@
 packages: .
+          ./examples
+          ./lens-properties
diff --git a/examples/lens-examples.cabal b/examples/lens-examples.cabal
--- a/examples/lens-examples.cabal
+++ b/examples/lens-examples.cabal
@@ -11,10 +11,12 @@
 bug-reports:   http://github.com/ekmett/lens/issues
 copyright:     Copyright (C) 2012 Edward A. Kmett
 synopsis:      Lenses, Folds and Traversals
-description:   Pong Example
+description:   Lenses, Folds and Traversals
+               .
+               Pong Example
 
 build-type:    Simple
-tested-with:   GHC == 7.4.1
+tested-with:   GHC == 7.4.2, GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.2, GHC == 8.4.1
 
 source-repository head
   type: git
@@ -28,7 +30,7 @@
     buildable: False
 
   build-depends:
-    base,
+    base       >= 4.5   && < 5,
     containers >= 0.4   && < 0.6,
     gloss      >= 1.7   && < 1.12,
     lens,
diff --git a/lens-properties/CHANGELOG.markdown b/lens-properties/CHANGELOG.markdown
--- a/lens-properties/CHANGELOG.markdown
+++ b/lens-properties/CHANGELOG.markdown
@@ -1,3 +1,7 @@
+4.11.1
+------
+* Update version bounds.
+
 4.0
 ---
 
diff --git a/lens.cabal b/lens.cabal
--- a/lens.cabal
+++ b/lens.cabal
@@ -1,6 +1,6 @@
 name:          lens
 category:      Data, Lenses, Generics
-version:       4.15.4
+version:       4.16
 license:       BSD2
 cabal-version: >= 1.8
 license-file:  LICENSE
@@ -12,7 +12,7 @@
 copyright:     Copyright (C) 2012-2016 Edward A. Kmett
 build-type:    Custom
 -- build-tools:   cpphs
-tested-with:   GHC == 7.4.2, GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.1
+tested-with:   GHC == 7.4.2, GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.2, GHC == 8.4.1
 synopsis:      Lenses, Folds and Traversals
 description:
   This package comes \"Batteries Included\" with many useful lenses for the types
@@ -195,7 +195,7 @@
     containers                >= 0.4.0    && < 0.6,
     distributive              >= 0.3      && < 1,
     filepath                  >= 1.2.0.0  && < 1.5,
-    free                      >= 4        && < 5,
+    free                      >= 4        && < 6,
     ghc-prim,
     hashable                  >= 1.1.2.3  && < 1.3,
     kan-extensions            >= 5        && < 6,
@@ -219,6 +219,9 @@
   if impl(ghc < 8.0)
     build-depends: generic-deriving >= 1.10 && < 2
 
+  if impl(ghc < 7.9)
+    build-depends: nats >= 0.1 && < 1.2
+
   exposed-modules:
     Control.Exception.Lens
     Control.Lens
@@ -268,6 +271,7 @@
     Control.Lens.Traversal
     Control.Lens.Tuple
     Control.Lens.Type
+    Control.Lens.Unsound
     Control.Lens.Wrapped
     Control.Lens.Zoom
     Control.Monad.Error.Lens
@@ -300,6 +304,7 @@
     System.IO.Error.Lens
     Language.Haskell.TH.Lens
     Numeric.Lens
+    Numeric.Natural.Lens
 
   other-modules:
     Paths_lens
diff --git a/src/Control/Lens/Combinators.hs b/src/Control/Lens/Combinators.hs
--- a/src/Control/Lens/Combinators.hs
+++ b/src/Control/Lens/Combinators.hs
@@ -1,3 +1,4 @@
+{-# Language CPP #-}
 --------------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Combinators
@@ -18,8 +19,7 @@
     ) where
 
 import Control.Lens hiding
-  ( -- output from scripts/operators
-    (<|)
+  ( (<|)
   , (|>)
   , (^..)
   , (^?)
@@ -91,6 +91,8 @@
   , (%%@=)
   , (<%@=)
   , (<<%@=)
+  , (.@=)
+  , (.@~)
   , (^#)
   , ( #~ )
   , ( #%~ )
@@ -109,6 +111,8 @@
   , (?~)
   , (<.~)
   , (<?~)
+  , (<<?~)
+  , (<<?=)
   , (+~)
   , (*~)
   , (-~)
@@ -137,4 +141,8 @@
   , (<>=)
   , (%@~)
   , (%@=)
+#if __GLASGOW_HASKELL__ >= 710
+  , (:>)
+  , (:<)
+#endif
   )
diff --git a/src/Control/Lens/Fold.hs b/src/Control/Lens/Fold.hs
--- a/src/Control/Lens/Fold.hs
+++ b/src/Control/Lens/Fold.hs
@@ -83,6 +83,7 @@
   , andOf, orOf
   , productOf, sumOf
   , traverseOf_, forOf_, sequenceAOf_
+  , traverse1Of_, for1Of_, sequence1Of_
   , mapMOf_, forMOf_, sequenceOf_
   , asumOf, msumOf
   , concatMapOf, concatOf
@@ -193,6 +194,7 @@
 -- >>> import Control.Exception (evaluate)
 -- >>> import Data.Maybe (fromMaybe)
 -- >>> import System.Timeout (timeout)
+-- >>> import qualified Data.Map as Map
 -- >>> let f :: Expr -> Expr; f = Debug.SimpleReflect.Vars.f
 -- >>> let g :: Expr -> Expr; g = Debug.SimpleReflect.Vars.g
 -- >>> let timingOut :: NFData a => a -> IO a; timingOut = fmap (fromMaybe (error "timeout")) . timeout (5*10^6) . evaluate . force
@@ -950,6 +952,48 @@
 sequenceAOf_ :: Functor f => Getting (Traversed a f) s (f a) -> s -> f ()
 sequenceAOf_ l = void . getTraversed #. foldMapOf l Traversed
 {-# INLINE sequenceAOf_ #-}
+
+-- | Traverse over all of the targets of a 'Fold1', computing an 'Apply' based answer.
+--
+-- As long as you have 'Applicative' or 'Functor' effect you are better using 'traverseOf_'.
+-- The 'traverse1Of_' is useful only when you have genuine 'Apply' effect.
+--
+-- >>> traverse1Of_ both1 (\ks -> Map.fromList [ (k, ()) | k <- ks ]) ("abc", "bcd")
+-- fromList [('b',()),('c',())]
+--
+-- @
+-- 'traverse1Of_' :: 'Apply' f => 'Fold1' s a -> (a -> f r) -> s -> f ()
+-- @
+--
+-- @since 4.16
+traverse1Of_ :: Functor f => Getting (TraversedF r f) s a -> (a -> f r) -> s -> f ()
+traverse1Of_ l f = void . getTraversedF #. foldMapOf l (TraversedF #. f)
+{-# INLINE traverse1Of_ #-}
+
+-- | See 'forOf_' and 'traverse1Of_'.
+--
+-- >>> for1Of_ both1 ("abc", "bcd") (\ks -> Map.fromList [ (k, ()) | k <- ks ])
+-- fromList [('b',()),('c',())]
+--
+-- @
+-- 'for1Of_' :: 'Apply' f => 'Fold1' s a -> s -> (a -> f r) -> f ()
+-- @
+--
+-- @since 4.16
+for1Of_ :: Functor f => Getting (TraversedF r f) s a -> s -> (a -> f r) -> f ()
+for1Of_ = flip . traverse1Of_
+{-# INLINE for1Of_ #-}
+
+-- | See 'sequenceAOf_' and 'traverse1Of_'.
+--
+-- @
+-- 'sequence1Of_' :: 'Apply' f => 'Fold1' s (f a) -> s -> f ()
+-- @
+--
+-- @since 4.16
+sequence1Of_ :: Functor f => Getting (TraversedF a f) s (f a) -> s -> f ()
+sequence1Of_ l = void . getTraversedF #. foldMapOf l TraversedF
+{-# INLINE sequence1Of_ #-}
 
 -- | Map each target of a 'Fold' on a structure to a monadic action, evaluate these actions from left to right, and ignore the results.
 --
diff --git a/src/Control/Lens/Internal/Fold.hs b/src/Control/Lens/Internal/Fold.hs
--- a/src/Control/Lens/Internal/Fold.hs
+++ b/src/Control/Lens/Internal/Fold.hs
@@ -21,6 +21,7 @@
   -- * Monoids for folding
     Folding(..)
   , Traversed(..)
+  , TraversedF(..)
   , Sequenced(..)
   , Max(..), getMax
   , Min(..), getMin
@@ -53,8 +54,8 @@
 -- | A 'Monoid' for a 'Contravariant' 'Applicative'.
 newtype Folding f a = Folding { getFolding :: f a }
 
-instance (Contravariant f, Apply f) => Semigroup (Folding f a) where
-  Folding fr <> Folding fs = Folding (fr .> fs)
+instance (Contravariant f, Applicative f) => Semigroup (Folding f a) where
+  Folding fr <> Folding fs = Folding (fr *> fs)
   {-# INLINE (<>) #-}
 
 instance (Contravariant f, Applicative f) => Monoid (Folding f a) where
@@ -72,8 +73,9 @@
 -- The argument 'a' of the result should not be used!
 newtype Traversed a f = Traversed { getTraversed :: f a }
 
-instance Apply f => Semigroup (Traversed a f) where
-  Traversed ma <> Traversed mb = Traversed (ma .> mb)
+-- See 4.16 Changelog entry for the explanation of "why not Apply f =>"?
+instance Applicative f => Semigroup (Traversed a f) where
+  Traversed ma <> Traversed mb = Traversed (ma *> mb)
   {-# INLINE (<>) #-}
 
 instance Applicative f => Monoid (Traversed a f) where
@@ -83,16 +85,37 @@
   {-# INLINE mappend #-}
 
 ------------------------------------------------------------------------------
+-- TraversedF
+------------------------------------------------------------------------------
+
+-- | Used internally by 'Control.Lens.Fold.traverse1Of_' and the like.
+--
+-- @since 4.16
+newtype TraversedF a f = TraversedF { getTraversedF :: f a }
+
+instance Apply f => Semigroup (TraversedF a f) where
+  TraversedF ma <> TraversedF mb = TraversedF (ma .> mb)
+  {-# INLINE (<>) #-}
+
+instance (Apply f, Applicative f) => Monoid (TraversedF a f) where
+  mempty = TraversedF (pure (error "TraversedF: value used"))
+  {-# INLINE mempty #-}
+  TraversedF ma `mappend` TraversedF mb = TraversedF (ma *> mb)
+  {-# INLINE mappend #-}
+
+------------------------------------------------------------------------------
 -- Sequenced
 ------------------------------------------------------------------------------
 
 -- | Used internally by 'Control.Lens.Traversal.mapM_' and the like.
 --
 -- The argument 'a' of the result should not be used!
+--
+-- See 4.16 Changelog entry for the explanation of "why not Apply f =>"?
 newtype Sequenced a m = Sequenced { getSequenced :: m a }
 
-instance Apply m => Semigroup (Sequenced a m) where
-  Sequenced ma <> Sequenced mb = Sequenced (ma .> mb)
+instance Monad m => Semigroup (Sequenced a m) where
+  Sequenced ma <> Sequenced mb = Sequenced (ma >> mb)
   {-# INLINE (<>) #-}
 
 instance Monad m => Monoid (Sequenced a m) where
diff --git a/src/Control/Lens/Internal/Indexed.hs b/src/Control/Lens/Internal/Indexed.hs
--- a/src/Control/Lens/Internal/Indexed.hs
+++ b/src/Control/Lens/Internal/Indexed.hs
@@ -50,10 +50,12 @@
 import Data.Functor.Bind
 import Data.Functor.Contravariant
 import Data.Int
+import Data.Monoid
 import Data.Profunctor.Closed
 import Data.Profunctor
 import Data.Profunctor.Rep
 import Data.Profunctor.Sieve
+import qualified Data.Semigroup as Semi
 import Data.Traversable
 import Prelude hiding ((.),id)
 #ifndef SAFE
@@ -275,6 +277,28 @@
   contramap f (Indexing m) = Indexing $ \i -> case m i of
     (j, ff) -> (j, contramap f ff)
   {-# INLINE contramap #-}
+
+instance Semi.Semigroup (f a) => Semi.Semigroup (Indexing f a) where
+    Indexing mx <> Indexing my = Indexing $ \i -> case mx i of
+      (j, x) -> case my j of
+         ~(k, y) -> (k, x Semi.<> y)
+    {-# INLINE (<>) #-}
+
+-- |
+--
+-- >>> "cat" ^@.. (folded <> folded)
+-- [(0,'c'),(1,'a'),(2,'t'),(0,'c'),(1,'a'),(2,'t')]
+--
+-- >>> "cat" ^@.. indexing (folded <> folded)
+-- [(0,'c'),(1,'a'),(2,'t'),(3,'c'),(4,'a'),(5,'t')]
+instance Monoid (f a) => Monoid (Indexing f a) where
+    mempty = Indexing $ \i -> (i, mempty)
+    {-# INLINE mempty #-}
+
+    mappend (Indexing mx) (Indexing my) = Indexing $ \i -> case mx i of
+      (j, x) -> case my j of
+         ~(k, y) -> (k, mappend x y)
+    {-# INLINE mappend #-}
 
 -- | Transform a 'Control.Lens.Traversal.Traversal' into an 'Control.Lens.Traversal.IndexedTraversal' or
 -- a 'Control.Lens.Fold.Fold' into an 'Control.Lens.Fold.IndexedFold', etc.
diff --git a/src/Control/Lens/Internal/Zoom.hs b/src/Control/Lens/Internal/Zoom.hs
--- a/src/Control/Lens/Internal/Zoom.hs
+++ b/src/Control/Lens/Internal/Zoom.hs
@@ -287,8 +287,8 @@
   contramap _ (Effect m) = Effect m
   {-# INLINE contramap #-}
 
-instance (Apply m, Semigroup r) => Semigroup (Effect m r a) where
-  Effect ma <> Effect mb = Effect (liftF2 (<>) ma mb)
+instance (Monad m, Semigroup r) => Semigroup (Effect m r a) where
+  Effect ma <> Effect mb = Effect (liftM2 (<>) ma mb)
   {-# INLINE (<>) #-}
 
 instance (Monad m, Monoid r) => Monoid (Effect m r a) where
diff --git a/src/Control/Lens/Iso.hs b/src/Control/Lens/Iso.hs
--- a/src/Control/Lens/Iso.hs
+++ b/src/Control/Lens/Iso.hs
@@ -104,6 +104,8 @@
 import Control.Monad.Writer.Strict as Strict
 import Control.Monad.RWS.Lazy as Lazy
 import Control.Monad.RWS.Strict as Strict
+import Control.Monad.ST.Lazy as Lazy
+import Control.Monad.ST as Strict
 import Data.Bifunctor
 import Data.ByteString as StrictB hiding (reverse)
 import Data.ByteString.Lazy as LazyB hiding (reverse)
@@ -449,6 +451,10 @@
 
 instance Strict (Lazy.RWST r w s m a) (Strict.RWST r w s m a) where
   strict = iso (Strict.RWST . Lazy.runRWST) (Lazy.RWST . Strict.runRWST)
+  {-# INLINE strict #-}
+
+instance Strict (Lazy.ST s a) (Strict.ST s a) where
+  strict = iso Lazy.lazyToStrictST Lazy.strictToLazyST
   {-# INLINE strict #-}
 
 -- | An 'Iso' between the strict variant of a structure and its lazy
diff --git a/src/Control/Lens/Reified.hs b/src/Control/Lens/Reified.hs
--- a/src/Control/Lens/Reified.hs
+++ b/src/Control/Lens/Reified.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE Rank2Types #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleInstances #-}
@@ -37,7 +38,9 @@
 import Data.Profunctor
 import Data.Profunctor.Rep
 import Data.Profunctor.Sieve
+#if !(MIN_VERSION_base(4,11,0))
 import Data.Semigroup
+#endif
 
 -- $setup
 -- >>> import Control.Lens
diff --git a/src/Control/Lens/Traversal.hs b/src/Control/Lens/Traversal.hs
--- a/src/Control/Lens/Traversal.hs
+++ b/src/Control/Lens/Traversal.hs
@@ -230,11 +230,11 @@
 --
 --  * a 'Traversal' if @f@ is 'Applicative',
 --
---  * a 'Getter' if @f@ is only a 'Functor' and 'Contravariant',
+--  * a 'Getter' if @f@ is only a 'Functor' and 'Data.Functor.Contravariant.Contravariant',
 --
---  * a 'Lens' if @p@ is only a 'Functor',
+--  * a 'Lens' if @f@ is only a 'Functor',
 --
---  * a 'Fold' if @f@ is 'Functor', 'Contravariant' and 'Applicative'.
+--  * a 'Fold' if @f@ is 'Applicative' and 'Data.Functor.Contravariant.Contravariant'.
 type Traversing p f s t a b = Over p (BazaarT p f a b) s t a b
 
 type Traversing1 p f s t a b = Over p (BazaarT1 p f a b) s t a b
@@ -606,7 +606,7 @@
 -- Given a context you can use 'Control.Comonad.Store.Class.pos' to see the values, 'Control.Comonad.Store.Class.peek' at what the structure would be like with an edited result, or simply 'extract' the original structure.
 --
 -- @
--- propChildren l x = childrenOf l x '==' 'map' 'Control.Comonad.Store.Class.pos' ('holesOf' l x)
+-- propChildren l x = 'toListOf' l x '==' 'map' 'Control.Comonad.Store.Class.pos' ('holesOf' l x)
 -- propId l x = 'all' ('==' x) ['extract' w | w <- 'holesOf' l x]
 -- @
 --
@@ -1274,9 +1274,9 @@
 deepOf :: (Conjoined p, Applicative f) => LensLike f s t s t -> Traversing p f s t a b -> Over p f s t a b
 deepOf r l = failing l (r . deepOf r l)
 
--- | "Fuse" a 'Traversal' by reassociating all of the '\<*\>' operations to the
+-- | "Fuse" a 'Traversal' by reassociating all of the '<*>' operations to the
 -- left and fusing all of the 'fmap' calls into one. This is particularly
--- useful when constructing a 'Traversal' using operations from GHC.Generics.
+-- useful when constructing a 'Traversal' using operations from "GHC.Generics".
 --
 -- Given a pair of 'Traversal's 'foo' and 'bar',
 --
diff --git a/src/Control/Lens/Unsound.hs b/src/Control/Lens/Unsound.hs
new file mode 100644
--- /dev/null
+++ b/src/Control/Lens/Unsound.hs
@@ -0,0 +1,93 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# OPTIONS_GHC -fno-warn-warnings-deprecations #-}
+
+#ifndef MIN_VERSION_mtl
+#define MIN_VERSION_mtl(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708
+{-# LANGUAGE Trustworthy #-}
+#endif
+
+{-# LANGUAGE RankNTypes #-}
+
+-------------------------------------------------------------------------------
+-- |
+-- Module      :  Control.Lens.Unsound
+-- Copyright   :  (C) 2012-16 Edward Kmett
+-- License     :  BSD-style (see the file LICENSE)
+-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
+-- Stability   :  provisional
+-- Portability :  Rank2Types
+--
+-- One commonly asked question is: can we combine two lenses,
+-- @`Lens'` a b@ and @`Lens'` a c@ into @`Lens'` a (b, c)@.
+-- This is fair thing to ask, but such operation is unsound in general. 
+-- See `lensProduct`.
+--
+-------------------------------------------------------------------------------
+module Control.Lens.Unsound
+  ( 
+    lensProduct
+  , prismSum
+  ) where
+
+import Control.Applicative
+import Control.Lens
+import Prelude
+
+-- | A lens product. There is no law-abiding way to do this in general.
+-- Result is only a valid 'Lens' if the input lenses project disjoint parts of 
+-- the structure @s@. Otherwise "you get what you put in" law
+--
+-- @
+-- 'Control.Lens.Getter.view' l ('Control.Lens.Setter.set' l v s) ≡ v
+-- @
+--
+-- is violated by
+--
+-- >>> let badLens :: Lens' (Int, Char) (Int, Int); badLens = lensProduct _1 _1
+-- >>> view badLens (set badLens (1,2) (3,'x'))
+-- (2,2)
+--
+-- but we should get @(1,2)@.
+--
+-- Are you looking for 'Control.Lens.Lens.alongside'?
+--
+lensProduct :: ALens' s a -> ALens' s b -> Lens' s (a, b)
+lensProduct l1 l2 f s =
+    f (s ^# l1, s ^# l2) <&> \(a, b) -> s & l1 #~ a & l2 #~ b
+
+-- | A dual of `lensProduct`: a prism sum.
+--
+-- The law
+--
+-- @
+-- 'Control.Lens.Fold.preview' l ('Control.Lens.Review.review' l b) ≡ 'Just' b
+-- @
+--
+-- breaks with
+--
+-- >>> let badPrism :: Prism' (Maybe Char) (Either Char Char); badPrism = prismSum _Just _Just
+-- >>> preview badPrism (review badPrism (Right 'x'))
+-- Just (Left 'x')
+--
+-- We put in 'Right' value, but get back 'Left'.
+-- 
+-- Are you looking for 'Control.Lens.Prism.without'?
+--
+prismSum :: APrism s t a b
+         -> APrism s t c d
+         -> Prism s t (Either a c) (Either b d)
+prismSum k =
+    withPrism k                  $ \bt seta k' ->
+    withPrism k'                 $ \dt setb    ->
+    prism (either bt dt) $ \s ->
+    f (Left <$> seta s) (Right <$> setb s)
+  where
+    f a@(Right _) _ = a
+    f (Left _)    b = b 
diff --git a/src/Language/Haskell/TH/Lens.hs b/src/Language/Haskell/TH/Lens.hs
--- a/src/Language/Haskell/TH/Lens.hs
+++ b/src/Language/Haskell/TH/Lens.hs
@@ -229,9 +229,11 @@
 #endif
   -- ** FunDep Prisms TODO make a lens
   , _FunDep
+#if !(MIN_VERSION_template_haskell(2,13,0))
   -- ** FamFlavour Prisms
   , _TypeFam
   , _DataFam
+#endif
   -- ** FixityDirection Prisms
   , _InfixL
   , _InfixR
@@ -275,6 +277,9 @@
 #if MIN_VERSION_template_haskell(2,11,0)
   , _UnboundVarE
 #endif
+#if MIN_VERSION_template_haskell(2,13,0)
+  , _LabelE
+#endif
   -- ** Body Prisms
   , _GuardedB
   , _NormalB
@@ -1689,6 +1694,7 @@
       reviewer (x, y) = FunDep x y
       remitter (FunDep x y) = (x, y)
 
+#if !(MIN_VERSION_template_haskell(2,13,0))
 _TypeFam :: Prism' FamFlavour ()
 _TypeFam
   = prism' reviewer remitter
@@ -1704,6 +1710,7 @@
       reviewer () = DataFam
       remitter DataFam = Just ()
       remitter _ = Nothing
+#endif
 
 #if MIN_VERSION_template_haskell(2,9,0)
 tySynEqnPatterns :: Lens' TySynEqn [Type]
@@ -1958,6 +1965,16 @@
   where
       reviewer = UnboundVarE
       remitter (UnboundVarE x) = Just x
+      remitter _ = Nothing
+#endif
+
+#if MIN_VERSION_template_haskell(2,13,0)
+_LabelE :: Prism' Exp String
+_LabelE
+  = prism' reviewer remitter
+  where
+      reviewer = LabelE
+      remitter (LabelE x) = Just x
       remitter _ = Nothing
 #endif
 
diff --git a/src/Numeric/Natural/Lens.hs b/src/Numeric/Natural/Lens.hs
new file mode 100644
--- /dev/null
+++ b/src/Numeric/Natural/Lens.hs
@@ -0,0 +1,93 @@
+{-# language CPP #-}
+{-# language RankNTypes #-}
+{-# language PatternGuards #-}
+#if __GLASGOW_HASKELL__ >= 710
+{-# language ViewPatterns #-}
+{-# language PatternSynonyms #-}
+#endif
+--------------------------------------------------------------------------------
+-- |
+-- Module      :  Numeric.Natural.Lens
+-- Copyright   :  (C) 2017 Edward Kmett
+-- License     :  BSD-style (see the file LICENSE)
+-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
+-- Stability   :  provisional
+-- Portability :  portable
+--
+-- Useful tools for Gödel numbering.
+-------------------------------------------------------------------------------
+module Numeric.Natural.Lens
+  ( _Pair
+  , _Sum
+  , _Naturals
+#if __GLASGOW_HASKELL__ >= 710
+  , pattern Pair
+  , pattern Sum
+  , pattern Naturals
+#endif
+  ) where
+
+import Control.Lens
+import Numeric.Natural
+
+-- | The natural numbers are isomorphic to the product of the natural numbers with itself.
+--
+-- @N = N*N@
+_Pair :: Iso' Natural (Natural, Natural)
+_Pair = iso hither (uncurry yon) where
+  yon 0 0 = 0
+  yon m n = case quotRem m 2 of
+    (q,r) -> r + 2 * yon n q -- rotation
+
+  hither 0 = (0,0)
+  hither n = case quotRem n 2 of
+   (p,r) -> case hither p of
+     (x,y) -> (r+2*y,x) -- rotation
+
+-- | The natural numbers are isomorphic to disjoint sums of natural numbers embedded as
+-- evens or odds.
+--
+-- @N = 2*N@
+_Sum :: Iso' Natural (Either Natural Natural)
+_Sum = iso hither yon where
+  hither p = case quotRem p 2 of
+    (q,0) -> Left q
+    (q,1) -> Right q
+    _     -> error "_Sum: impossible"
+  yon (Left q)  = 2*q
+  yon (Right q) = 2*q+1
+
+-- | The natural numbers are isomorphic to lists of natural numbers
+_Naturals :: Iso' Natural [Natural]
+_Naturals = iso hither yon where
+  hither 0 = []
+  hither n | (h, t) <- (n-1)^._Pair = h : hither t
+  yon [] = 0
+  yon (x:xs) = 1 + review _Pair (x, yon xs)
+
+#if __GLASGOW_HASKELL__ >= 710
+
+-- |
+-- interleaves the bits of two natural numbers
+pattern Pair :: Natural -> Natural -> Natural
+pattern Pair x y <- (view _Pair -> (x,y)) where
+  Pair x y = review _Pair (x,y)
+
+-- |
+-- @
+-- Sum (Left q) = 2*q
+-- Sum (Right q) = 2*q+1
+-- @
+pattern Sum :: Either Natural Natural -> Natural
+pattern Sum s <- (view _Sum -> s) where
+  Sum s = review _Sum s
+
+-- |
+-- @
+-- Naturals [] = 0
+-- Naturals (h:t) = 1 + Pair h (Naturals t)
+-- @
+pattern Naturals :: [Natural] -> Natural
+pattern Naturals xs <- (view _Naturals -> xs) where
+  Naturals xs = review _Naturals xs
+#endif