diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,14 @@
+# Revision history for multiwalk
+
+## 0.3.0.1 -- 2023-07-17
+* Improve haddocks.
+
+## 0.3.0.0 -- 2023-07-08
+* Add `Spec` kind, in particular `SpecSelf` for matching the type itself.
+
+## 0.2.0.0 -- 2023-07-07
+* Change order of type arguments for `walkM`
+
+## 0.1.0.0 -- 2023-04-05
+
+* First version. Released on an unsuspecting world.
diff --git a/LICENSE b/LICENSE
new file mode 100644
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,674 @@
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+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+              END OF TERMS AND CONDITIONS
+
+     How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<http://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<http://www.gnu.org/philosophy/why-not-lgpl.html>.
diff --git a/README.md b/README.md
new file mode 100644
--- /dev/null
+++ b/README.md
@@ -0,0 +1,50 @@
+# MultiWalk
+
+This library provides functionality for traversing data types recursively,
+acting on multiple types during the same traversal. In spirit, it is similar to
+the `Walk` type class from `Pandoc.Walk`, but generalizes it by allowing
+multiple types to be targeted by the traversal. Also, by default it only
+requires an `Applicative` constraint on the action, making it suitable for
+situations where you don't have a `Monad` (for instance, traversing within a
+composition of monads).
+
+Say, for instance, you want to query all code snippets from a Pandoc document,
+including both `Inline` and `Block` ones, such that the list of results is *in
+the same order* as they appear in the document. There is no way to do this with
+Pandoc's `Walk` type class, because it only supports querying or walking with
+one type at once. With MultiWalk you are able to do this with a single query
+function that targets both types, and it'll look something like this:
+
+```haskell
+multi :: Block -> [Text]
+multi = buildMultiQ @PTag $ \sub list ->
+    list ?> blks sub
+         ?> inls sub
+  where
+    blks _ (CodeBlock _ c) = [c]
+    blks f x = f x
+    inls _ (Code _ c) = [c]
+    inls f x = f x
+```
+
+(note, however, that this library does not ship with default instances for
+Pandoc, so you will have to define them yourself. You can find a basic Pandoc
+instance for reference, and the function above, in `Benchmark.hs` inside the
+test directory.)
+
+Another use case is when you want to modify a data type, perhaps targeting
+multiple subtypes, and you want to do that inside a functor that is
+`Applicative` but not a `Monad`. Such functors may sound unusual, but one of the
+interesting places where they appear are in composition of monads, which need
+not be a monad itself. 
+
+You can find a use of such monad compositions in my other library
+[here](https://github.com/lucasvreis/org-mode-hs/blob/main/org-exporters/src/Org/Exporters/Processing/InternalLinks.hs).
+It's used for resolving cross-references inside documents inside the functor
+`Compose M F`, where `M` is a state monad which holds the links it has found so
+far, and `F` is a reader which receives the _final, future state_ (that will
+only be ready _at the end of the computation_). In this way I can walk across
+the document only once, registering and applying cross references at the same
+time. Note that this is composition is semantically different from using monad
+transformers, because they would require the reader state to be supplied before
+the final state is available.
diff --git a/multiwalk.cabal b/multiwalk.cabal
new file mode 100644
--- /dev/null
+++ b/multiwalk.cabal
@@ -0,0 +1,159 @@
+cabal-version:   3.0
+
+-- The cabal-version field refers to the version of the .cabal specification,
+-- and can be different from the cabal-install (the tool) version and the
+-- Cabal (the library) version you are using. As such, the Cabal (the library)
+-- version used must be equal or greater than the version stated in this field.
+-- Starting from the specification version 2.2, the cabal-version field must be
+-- the first thing in the cabal file.
+
+-- Initial package description 'multiwalk' generated by
+-- 'cabal init'. For further documentation, see:
+--   http://haskell.org/cabal/users-guide/
+--
+-- The name of the package.
+name:            multiwalk
+
+-- The package version.
+-- See the Haskell package versioning policy (PVP) for standards
+-- guiding when and how versions should be incremented.
+-- https://pvp.haskell.org
+-- PVP summary:     +-+------- breaking API changes
+--                  | | +----- non-breaking API additions
+--                  | | | +--- code changes with no API change
+version:         0.3.0.1
+
+-- A short (one-line) description of the package.
+synopsis:
+  Traverse data types via generics, acting on multiple types simultaneously.
+
+-- A longer description of the package.
+description:
+  This library provides functionality for traversing data types recursively,
+  acting on multiple types during the same traversal. In spirit, it is similar to
+  the Walk type class from Pandoc.Walk, but generalizes it by allowing
+  multiple types to be targeted by the traversal. In general, it only
+  requires an Applicative constraint on the action, making it suitable for
+  situations where you don't have a Monad.
+
+-- The license under which the package is released.
+license:         GPL-3.0-or-later
+
+-- The file containing the license text.
+license-file:    LICENSE
+
+-- The package author(s).
+author:          Lucas V. R.
+
+-- An email address to which users can send suggestions, bug reports, and patches.
+maintainer:      @lucasvr:matrix.org
+
+-- A copyright notice.
+-- copyright:
+category:        Control
+build-type:      Simple
+
+-- Extra doc files to be distributed with the package, such as a CHANGELOG or a README.
+extra-doc-files:
+  CHANGELOG.md
+  README.md
+
+-- Extra source files to be distributed with the package, such as examples, or a tutorial module.
+-- extra-source-files:
+
+source-repository head
+  type:     git
+  location: https://github.com/lucasvreis/multiwalk
+
+common common-options
+  ghc-options:        -Wall -fprint-potential-instances
+  default-extensions:
+    AllowAmbiguousTypes
+    ConstraintKinds
+    DataKinds
+    DefaultSignatures
+    DeriveGeneric
+    FlexibleContexts
+    FlexibleInstances
+    KindSignatures
+    LambdaCase
+    MultiParamTypeClasses
+    ScopedTypeVariables
+    TypeApplications
+    TypeFamilies
+    TypeOperators
+    UndecidableInstances
+
+library
+  import:           common-options
+
+  -- Modules exported by the library.
+  exposed-modules:
+    Control.MultiWalk
+    Control.MultiWalk.Contains
+    Control.MultiWalk.HasSub
+
+  -- Modules included in this library but not exported.
+  -- other-modules:
+
+  -- LANGUAGE extensions used by modules in this package.
+  -- other-extensions:
+
+  -- Other library packages from which modules are imported.
+  build-depends:    base >=4.16 && <4.19
+
+  -- Directories containing source files.
+  hs-source-dirs:   src
+
+  -- Base language which the package is written in.
+  default-language: Haskell2010
+
+test-suite multiwalk-test
+  import:           common-options
+
+  -- Base language which the package is written in.
+  default-language: Haskell2010
+
+  -- Modules included in this executable, other than Main.
+  -- other-modules:
+
+  -- LANGUAGE extensions used by modules in this package.
+  -- other-extensions:
+
+  -- The interface type and version of the test suite.
+  type:             exitcode-stdio-1.0
+
+  -- Directories containing source files.
+  hs-source-dirs:   test
+
+  -- The entrypoint to the test suite.
+  main-is:          Main.hs
+
+  -- Test dependencies.
+  build-depends:
+    , base
+    , multiwalk
+    , tasty        >=1.4.3  && <1.5
+    , tasty-hunit  >=0.10.0 && <0.11
+
+benchmark bench
+  import:           common-options
+  hs-source-dirs:   test
+  main-is:          Benchmark.hs
+  type:             exitcode-stdio-1.0
+  build-depends:
+    , base
+    , bytestring         >=0.11.4 && <0.12
+    , commonmark         >=0.2.2  && <0.3
+    , commonmark-pandoc  >=0.2.1  && <0.3
+    , deepseq            >=1.4.6  && <1.5
+    , multiwalk
+    , pandoc-types       >=1.23   && <1.24
+    , tasty
+    , tasty-bench        >=0.3.3  && <0.4
+    , tasty-hunit
+    , text               >=1.2.5  && <2.1
+    , transformers       >=0.5.6  && <0.7
+
+  ghc-options:      "-with-rtsopts=-A32m --nonmoving-gc"
+  default-language: Haskell2010
diff --git a/src/Control/MultiWalk.hs b/src/Control/MultiWalk.hs
new file mode 100644
--- /dev/null
+++ b/src/Control/MultiWalk.hs
@@ -0,0 +1,239 @@
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE UndecidableSuperClasses #-}
+
+{- | This module provides functionality for recursively traversing and querying
+   along multiple types.
+-}
+module Control.MultiWalk (
+  MultiTag (..),
+  MultiSub (..),
+  query,
+  walk,
+  walkM,
+  Walk,
+  Query,
+  walkSub,
+  querySub,
+  buildMultiW,
+  buildMultiQ,
+  (.>),
+  (?>),
+  ToSpec,
+  ToSpecSel,
+  Spec (..),
+  SelSpec (..),
+  Trav,
+  MatchWith,
+  Under,
+  MultiWalk,
+  FList (..),
+  QList (..),
+  BuildF (..),
+  BuildQ (..),
+)
+where
+
+import Control.Monad ((>=>))
+import Control.MultiWalk.Contains
+import Data.Functor.Identity (Identity (Identity, runIdentity))
+import Data.Kind (Type)
+
+{- | You should instantiate 'MultiTag' to a tag associated to the structure you
+   are working with. The tag is mostly there to prevent orphan instances, since
+   people are often working with structures from other packages (Pandoc AST,
+   HTML, etc.)
+-}
+class
+  ( BuildF (MultiWalk' tag) (MultiTypes tag)
+  , BuildQ (MultiWalk' tag) (MultiTypes tag)
+  ) =>
+  MultiTag tag
+  where
+  -- | The types that will be used in the walks and queries; every type listed here
+  --    should have a 'MultiSub' instance. (The compiler will complain about this.)
+  type MultiTypes tag :: [Type]
+
+class MultiSub tag t where
+  -- | A list of substructure specifications for types that are substructures to
+  -- this type; all types listed here should also be listed in the corresponding
+  -- 'MultiTypes', but you can omit types from there that should not be regarded
+  -- as subtypes.
+  --
+  -- Substructure specifications are special datakinds that you can generate
+  -- using 'ToSpec' and 'ToSpecSel', and the combinators (eg. 'Under',
+  -- 'MatchWith' and 'Trav').
+  type SubTypes tag t :: Spec
+
+  -- | If you want to write HasSub instances by hand (not that easy), you can
+  -- put the associated HasSub tag here. Defaults to 'GSubTag' (which derives
+  -- 'Generic' instances).
+  type HasSubTag tag t :: Type
+
+  type HasSubTag tag t = GSubTag
+
+type MultiWalk tag t =
+  ( AllMods (TContains (MultiTypes tag)) (SubTypes tag t)
+  , QContains (MultiTypes tag) t
+  , FContains (MultiTypes tag) t
+  , HasSub (HasSubTag tag t) (SubTypes tag t) t
+  )
+
+class (MultiWalk tag t) => MultiWalk' tag t
+
+instance (MultiWalk tag t) => MultiWalk' tag t
+
+-- | Query (only) substructures by applying functions from 'QList'.
+querySub :: forall tag t m. (Monoid m, MultiWalk tag t) => QList m (MultiTypes tag) -> t -> m
+querySub = getSubWithQList @(HasSubTag tag t) @(SubTypes tag t)
+
+-- | Modify (only) substructures by applying functions from 'FList'.
+walkSub :: forall tag t m. (Applicative m, MultiWalk tag t) => FList m (MultiTypes tag) -> t -> m t
+walkSub = modSubWithFList @(HasSubTag tag t) @(SubTypes tag t)
+
+-- | Query a structure with a single query function (just like Pandoc.Walk).
+query ::
+  forall tag m t a.
+  ( MultiTag tag
+  , MultiWalk tag a
+  , MultiWalk tag t
+  , Monoid m
+  ) =>
+  (t -> m) ->
+  a ->
+  m
+query f =
+  buildMultiQ @tag $ \go l ->
+    l ?> \x -> f x <> go x
+
+-- | Modify a structure by walking with a single function (just like Pandoc.Walk).
+walk ::
+  forall tag t c.
+  (MultiTag tag, MultiWalk tag c, MultiWalk tag t) =>
+  (t -> t) ->
+  c ->
+  c
+walk f = runIdentity . walkM @tag (Identity . f)
+
+-- | Modify a structure by walking with a single function (just like Pandoc.Walk).
+walkM ::
+  forall tag t a m.
+  ( Monad m
+  , MultiTag tag
+  , MultiWalk tag a
+  , MultiWalk tag t
+  ) =>
+  (t -> m t) ->
+  a ->
+  m a
+walkM f =
+  buildMultiW @tag $ \go l ->
+    l .> (f >=> go)
+
+type Query tag m = forall t. MultiWalk tag t => t -> m
+
+{- | Most general way to create a query. Create a query with multiple functions,
+   targeting multiple types.
+
+   First argument is a function that takes a query, an empty list of queries and
+   should return a list of queries populated with the multiple query functions.
+
+   By "tying a knot", the first argument you are supplied with is almost the
+   result of 'buildMultiQ' itself, the only difference being that it only
+   queries /substructures/ of the type. It's a responsability of each function
+   in the 'QList' to apply this function to its argument in any desired way, as
+   to continue recursing down the "type tree".
+
+   You can add functions to the empty 'QList' via '?>'.
+
+   > multi :: Block -> [Text]
+   > multi = buildMultiQ @PTag $ \sub list ->
+   >     list ?> blks sub
+   >          ?> inls sub
+   >   where
+   >     blks _ (CodeBlock _ c) = [c]
+   >     blks f x = f x
+   >     inls _ (Code _ c) = [c]
+   >     inls f x = f x
+-}
+buildMultiQ ::
+  forall tag m.
+  (MultiTag tag, Monoid m) =>
+  ( Query tag m ->
+    QList m (MultiTypes tag) ->
+    -- \^ Empty query that you should modify.
+    QList m (MultiTypes tag)
+  ) ->
+  Query tag m
+buildMultiQ f = qGet qlist
+  where
+    qlist :: QList m (MultiTypes tag)
+    qlist = f go $ buildQ @(MultiWalk' tag) go
+    go :: forall s. MultiWalk tag s => s -> m
+    go = querySub @tag qlist
+
+type Walk tag m = forall t. MultiWalk tag t => t -> m t
+
+{- | Most general way to create a walk. Create a walk with multiple functions,
+   targeting multiple types.
+
+   First argument is a function that takes a walk, an empty list of functions and
+   should return a list of functions populated with the multiple walk functions.
+
+   By "tying a knot", the first argument you are supplied with is almost the
+   result of 'buildMultiW' itself, the only difference being that it only walks
+   /substructures/ of the type. It's a responsability of each function in the
+   'FList' to apply this function to its argument in any desired way, as to
+   continue recursing down the "type tree".
+
+   You can add functions to the empty 'FList' via '.>'.
+
+   > multi :: Applicative m => Block -> m Block
+   > multi = buildMultiW @PTag $ \sub list ->
+   >     list .> blks sub
+   >          .> inls sub
+   >   where
+   >     blks _ (CodeBlock _ c) = Para [Str c]
+   >     blks f x = f x
+   >     inls _ (Code _ c) = Str c
+   >     inls f x = f x
+-}
+buildMultiW ::
+  forall tag m.
+  (MultiTag tag, Applicative m) =>
+  ( Walk tag m ->
+    FList m (MultiTypes tag) ->
+    FList m (MultiTypes tag)
+  ) ->
+  Walk tag m
+buildMultiW f = fGet flist
+  where
+    flist :: FList m (MultiTypes tag)
+    flist = f go $ buildF @(MultiWalk' tag) go
+    go :: forall s. MultiWalk tag s => s -> m s
+    go = walkSub @tag flist
+
+class All c ls => BuildQ c ls where
+  buildQ :: (forall t. c t => t -> m) -> QList m ls
+
+instance BuildQ c '[] where
+  buildQ _ = QNil
+
+instance (BuildQ c ls, c l) => BuildQ c (l : ls) where
+  buildQ f = f :?: buildQ @c @ls f
+
+class All c ls => BuildF c ls where
+  buildF :: (forall t. c t => t -> m t) -> FList m ls
+
+instance BuildF c '[] where
+  buildF _ = FNil
+
+instance (BuildF c ls, c l) => BuildF c (l : ls) where
+  buildF f = f :.: buildF @c @ls f
+
+-- | Add a function to a 'QList'.
+(?>) :: QContains ls t => QList m ls -> (t -> m) -> QList m ls
+(?>) = qSet
+
+-- | Add a function to a 'FList'.
+(.>) :: FContains ls t => FList m ls -> (t -> m t) -> FList m ls
+(.>) = fSet
diff --git a/src/Control/MultiWalk/Contains.hs b/src/Control/MultiWalk/Contains.hs
new file mode 100644
--- /dev/null
+++ b/src/Control/MultiWalk/Contains.hs
@@ -0,0 +1,149 @@
+{-# LANGUAGE GADTs #-}
+
+{- | This module contains the instances and definitions
+   supporting 'Control.Multiwalk' module, along with
+   the combinators for writing 'MultiSub' instances.
+-}
+module Control.MultiWalk.Contains (
+  module Control.MultiWalk.HasSub,
+  module Control.MultiWalk.Contains,
+) where
+
+import Control.MultiWalk.HasSub hiding (Carrier, HasSub (..), ToSpec, ToSpecSel)
+import qualified Control.MultiWalk.HasSub as HS
+import Data.Coerce (Coercible, coerce)
+import Data.Kind (Type)
+import Data.Proxy (Proxy (..))
+
+data MWCTag
+
+type family ContainsCarrier (a :: Type) :: Type where
+  ContainsCarrier (Under b s a) = b
+  ContainsCarrier (MatchWith s a) = s
+  ContainsCarrier (Trav f a) = f (Carrier a)
+  ContainsCarrier a = a
+
+type instance HS.Carrier MWCTag a = ContainsCarrier a
+type instance HS.Carrier MWCTag a = ContainsCarrier a
+
+type HasSub tag ls t = HS.HasSub MWCTag tag ls t
+type Carrier a = HS.Carrier MWCTag a
+type ToSpec a = HS.ToSpec MWCTag a
+type ToSpecSel s a = HS.ToSpecSel MWCTag s a
+
+-- | Modify (only) substructures by applying functions from 'FList'.
+modSubWithFList ::
+  forall tag ls t fs m.
+  (HasSub tag ls t, Applicative m, AllMods (TContains fs) ls) =>
+  FList m fs ->
+  t ->
+  m t
+modSubWithFList fs =
+  HS.modSub @MWCTag @tag @ls @t (Proxy @(TContains fs)) (\(_ :: Proxy s) -> tGetW @fs @s fs)
+
+-- | Query (only) substructures by applying functions from 'QList'.
+getSubWithQList ::
+  forall tag ls t fs m.
+  (HasSub tag ls t, Monoid m, AllMods (TContains fs) ls) =>
+  QList m fs ->
+  t ->
+  m
+getSubWithQList fs =
+  HS.getSub @MWCTag @tag @ls @t (Proxy @(TContains fs)) (\(_ :: Proxy s) -> tGetQ @fs @s fs)
+
+infixr 8 :?:
+
+-- | Heterogeneous list of queries
+data QList :: Type -> [Type] -> Type where
+  QNil :: QList m '[]
+  (:?:) :: (x -> m) -> QList m xs -> QList m (x : xs)
+
+class QContains (l :: [Type]) (t :: Type) where
+  qGet :: QList m l -> t -> m
+  qSet :: QList m l -> (t -> m) -> QList m l
+
+instance {-# OVERLAPS #-} QContains (t : xs) t where
+  qGet (f :?: _) = f
+  qSet (_ :?: y) z = z :?: y
+
+instance QContains xs t => QContains (x : xs) t where
+  qGet (_ :?: y) = qGet y
+  qSet (x :?: y) z = x :?: qSet y z
+
+infixr 8 :.:
+
+-- | Heterogeneous list of monadic-valued functions
+data FList :: (Type -> Type) -> [Type] -> Type where
+  FNil :: FList m '[]
+  (:.:) :: (x -> m x) -> FList m xs -> FList m (x : xs)
+
+class FContains (l :: [Type]) (t :: Type) where
+  fGet :: FList m l -> t -> m t
+  fSet :: FList m l -> (t -> m t) -> FList m l
+
+instance {-# OVERLAPS #-} FContains (t : xs) t where
+  fGet (f :.: _) = f
+  fSet (_ :.: y) z = z :.: y
+
+instance FContains xs t => FContains (x : xs) t where
+  fGet (_ :.: y) = fGet y
+  fSet (x :.: y) z = x :.: fSet y z
+
+{- | Auxiliary class that keeps track of how retrieve queries and walks from their
+   lists and apply them according to the combinators.
+-}
+class TContains (fs :: [Type]) (t :: Type) where
+  tGetW :: Applicative m => FList m fs -> ContainsCarrier t -> m (ContainsCarrier t)
+  tGetQ :: Monoid m => QList m fs -> ContainsCarrier t -> m
+
+instance
+  {-# OVERLAPPABLE #-}
+  ( FContains fs (Carrier a)
+  , QContains fs (Carrier a)
+  ) =>
+  TContains fs a
+  where
+  tGetW = fGet
+  tGetQ = qGet
+
+-- Useful combinators
+
+-- | Use this for matching with a type inside a traversable functor.
+data Trav (k :: Type -> Type) (a :: Type)
+
+instance
+  ( Traversable f
+  , TContains fs a
+  ) =>
+  TContains fs (Trav f a)
+  where
+  tGetW = traverse . tGetW @fs @a
+  tGetQ = foldMap . tGetQ @fs @a
+
+{- | Use this for matching with another type that is coercible to the type you
+   want.
+-}
+data MatchWith (s :: Type) (a :: Type)
+
+instance
+  ( TContains fs a
+  , Coercible (Carrier a) s
+  ) =>
+  TContains fs (MatchWith s a)
+  where
+  tGetW fs = fmap coerce . tGetW @fs @a fs . coerce
+  tGetQ fs = tGetQ @fs @a fs . coerce
+
+{- | Use this for matching a subcomponent nested inside another type. Useful if
+   you don't want to add the middle type to the list of walkable types.
+-}
+data Under (b :: Type) (s :: SelSpec) (a :: Type)
+
+instance
+  ( TContains fs a
+  , HasSub GSubTag ('SpecList '[ 'SubSpec s a (Carrier a)]) b
+  ) =>
+  TContains fs (Under b s a)
+  where
+  tGetW = modSubWithFList @GSubTag @('SpecList '[ 'SubSpec s a (Carrier a)]) @b @fs
+  tGetQ = getSubWithQList @GSubTag @('SpecList '[ 'SubSpec s a (Carrier a)]) @b @fs
diff --git a/src/Control/MultiWalk/HasSub.hs b/src/Control/MultiWalk/HasSub.hs
new file mode 100644
--- /dev/null
+++ b/src/Control/MultiWalk/HasSub.hs
@@ -0,0 +1,197 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+
+{- | This module is the heart of multiwalk, providing generic instances for
+   modifying and querying data types.
+-}
+module Control.MultiWalk.HasSub (
+  HasSub (..),
+  Spec (..),
+  SubSpec (..),
+  SelSpec (..),
+  Carrier,
+  ToSpec,
+  ToSpecSel,
+  All,
+  AllMods,
+  GSubTag,
+) where
+
+import Control.Applicative (liftA2)
+import Data.Kind (Constraint, Type)
+import Data.Proxy (Proxy (..))
+import GHC.Generics
+import GHC.TypeLits
+
+data Spec
+  = SpecList [SubSpec]
+  | SpecLeaf
+  | -- | Modifiers (used for customizing constraints)
+    SpecSelf
+      Type
+
+data SubSpec
+  = SubSpec
+      SelSpec
+      -- ^ Constructor and field selectors
+      Type
+      -- ^ Modifiers (used for customizing constraints)
+      Type
+      -- ^ Carrier type, should be equal to Carrier of type above (to be aligned
+      -- with the target's generic subtypes)
+
+type family Carrier ctag a :: Type
+
+data SelSpec
+  = NoSel
+  | ConsSel Symbol
+  | FieldSel Symbol
+  | ConsFieldSel Symbol Symbol
+
+type ToSpec tag (a :: Type) = 'SubSpec 'NoSel a (Carrier tag a)
+type ToSpecSel tag (s :: SelSpec) (a :: Type) = 'SubSpec s a (Carrier tag a)
+
+class HasSub ctag tag (ls :: Spec) t where
+  modSub ::
+    forall c m.
+    (Applicative m, AllMods c ls) =>
+    Proxy c ->
+    (forall s. c s => Proxy s -> Carrier ctag s -> m (Carrier ctag s)) ->
+    t ->
+    m t
+  getSub ::
+    forall c m.
+    (Monoid m, AllMods c ls) =>
+    Proxy c ->
+    (forall s. c s => Proxy s -> Carrier ctag s -> m) ->
+    t ->
+    m
+
+instance (Carrier tag s ~ t) => HasSub tag ctag ('SpecSelf s) t where
+  modSub _ f = f (Proxy @s)
+  getSub _ f = f (Proxy @s)
+
+instance HasSub tag ctag 'SpecLeaf t where
+  modSub _ _ = pure
+  getSub _ _ = mempty
+
+data GSubTag
+
+instance (Generic t, HasSub' ctag (l : ls) 'Nothing 'Nothing (Rep t)) => HasSub ctag GSubTag ('SpecList (l : ls)) t where
+  modSub p f = fmap to . modSub' @ctag @(l : ls) @'Nothing @'Nothing p f . from
+  getSub p f = getSub' @ctag @(l : ls) @'Nothing @'Nothing p f . from
+
+-- Generic code
+
+type family All (p :: k -> Constraint) (as :: [k]) :: Constraint where
+  All p '[] = ()
+  All p (a ': as) = (p a, All p as)
+
+type family SpecMods (s :: SubSpec) :: Type where
+  SpecMods ('SubSpec _ t _) = t
+
+type family AllMods (p :: Type -> Constraint) (as :: Spec) :: Constraint where
+  AllMods p ('SpecList ls) = AllMods' p ls
+  AllMods p ('SpecSelf t) = p t
+  AllMods p 'SpecLeaf = ()
+
+type family AllMods' (p :: Type -> Constraint) (as :: [SubSpec]) :: Constraint where
+  AllMods' p '[] = ()
+  AllMods' p (a ': as) = (p (SpecMods a), AllMods' p as)
+
+class HasSub' ctag (ls :: [SubSpec]) (cname :: Maybe Symbol) (sname :: Maybe Symbol) (t :: Type -> Type) where
+  modSub' ::
+    forall c m p.
+    (Applicative m, AllMods' c ls) =>
+    Proxy c ->
+    (forall s. c s => Proxy s -> Carrier ctag s -> m (Carrier ctag s)) ->
+    t p ->
+    m (t p)
+  getSub' ::
+    forall c m p.
+    (Monoid m, AllMods' c ls) =>
+    Proxy c ->
+    (forall s. c s => Proxy s -> Carrier ctag s -> m) ->
+    t p ->
+    m
+
+instance
+  Carrier ctag t1 ~ t2 =>
+  HasSub' ctag ('SubSpec 'NoSel t1 t2 : ls) _a _b (K1 _c t2)
+  where
+  modSub' _ f (K1 x) = K1 <$> f (Proxy @t1) x
+  getSub' _ f (K1 x) = f (Proxy @t1) x
+
+instance
+  Carrier ctag t1 ~ t2 =>
+  HasSub' ctag ('SubSpec ('FieldSel s) t1 t2 : ls) _a ('Just s) (K1 _c t2)
+  where
+  modSub' _ f (K1 x) = K1 <$> f (Proxy @t1) x
+  getSub' _ f (K1 x) = f (Proxy @t1) x
+
+instance
+  Carrier ctag t1 ~ t2 =>
+  HasSub' ctag ('SubSpec ('ConsSel s) t1 t2 : ls) ('Just s) _b (K1 _c t2)
+  where
+  modSub' _ f (K1 x) = K1 <$> f (Proxy @t1) x
+  getSub' _ f (K1 x) = f (Proxy @t1) x
+
+instance
+  Carrier ctag t1 ~ t2 =>
+  HasSub' ctag ('SubSpec ('ConsFieldSel s1 s2) t1 t2 : ls) ('Just s1) ('Just s2) (K1 _c t2)
+  where
+  modSub' _ f (K1 x) = K1 <$> f (Proxy @t1) x
+  getSub' _ f (K1 x) = f (Proxy @t1) x
+
+instance {-# OVERLAPPABLE #-} HasSub' ctag ls a b (K1 j s) => HasSub' ctag (l : ls) a b (K1 j s) where
+  modSub' = modSub' @ctag @ls @a @b
+  getSub' = getSub' @ctag @ls @a @b
+
+instance HasSub' ctag '[] _a _b (K1 _c _d) where
+  modSub' _ _ = pure
+  getSub' _ _ = mempty
+
+instance
+  ( HasSub' ctag s a 'Nothing x
+  , HasSub' ctag s a 'Nothing y
+  ) =>
+  HasSub' ctag s a 'Nothing (x :*: y)
+  where
+  modSub' p f (x :*: y) = liftA2 (:*:) (modSub' @ctag @s @a @'Nothing p f x) (modSub' @ctag @s @a @'Nothing p f y)
+  getSub' p f (x :*: y) = getSub' @ctag @s @a @'Nothing p f x <> getSub' @ctag @s @a @'Nothing p f y
+
+instance HasSub' ctag _a _b _c U1 where
+  modSub' _ _ = pure
+  getSub' _ _ = mempty
+
+instance
+  ( HasSub' ctag s 'Nothing 'Nothing x
+  , HasSub' ctag s 'Nothing 'Nothing y
+  ) =>
+  HasSub' ctag s 'Nothing 'Nothing (x :+: y)
+  where
+  modSub' p f (L1 x) = L1 <$> modSub' @ctag @s @'Nothing @'Nothing p f x
+  modSub' p f (R1 x) = R1 <$> modSub' @ctag @s @'Nothing @'Nothing p f x
+  getSub' p f (L1 x) = getSub' @ctag @s @'Nothing @'Nothing p f x
+  getSub' p f (R1 x) = getSub' @ctag @s @'Nothing @'Nothing p f x
+
+instance
+  HasSub' ctag ls a s x =>
+  HasSub' ctag ls a 'Nothing (S1 ('MetaSel s _a _b _c) x)
+  where
+  modSub' p f (M1 x) = M1 <$> modSub' @ctag @ls @a @s p f x
+  getSub' p f (M1 x) = getSub' @ctag @ls @a @s p f x
+
+instance
+  HasSub' ctag ls ('Just s) 'Nothing x =>
+  HasSub' ctag ls 'Nothing 'Nothing (C1 ('MetaCons s _a _b) x)
+  where
+  modSub' p f (M1 x) = M1 <$> modSub' @ctag @ls @('Just s) @'Nothing p f x
+  getSub' p f (M1 x) = getSub' @ctag @ls @('Just s) @'Nothing p f x
+
+instance
+  HasSub' ctag ls 'Nothing 'Nothing x =>
+  HasSub' ctag ls 'Nothing 'Nothing (D1 _a x)
+  where
+  modSub' p f (M1 x) = M1 <$> modSub' @ctag @ls @'Nothing @'Nothing p f x
+  getSub' p f (M1 x) = getSub' @ctag @ls @'Nothing @'Nothing p f x
diff --git a/test/Benchmark.hs b/test/Benchmark.hs
new file mode 100644
--- /dev/null
+++ b/test/Benchmark.hs
@@ -0,0 +1,151 @@
+{-# LANGUAGE RankNTypes #-}
+
+module Main where
+
+import Commonmark.Pandoc
+import Commonmark.Parser
+import Control.DeepSeq
+import Control.Exception (evaluate)
+import qualified Control.Monad.Trans.Writer.Lazy as LW
+import qualified Control.Monad.Trans.Writer.Strict as CPSW
+import qualified Control.Monad.Trans.Writer.Strict as SW
+import Control.MultiWalk
+import qualified Data.ByteString as B
+import Data.Functor (($>))
+import Data.Functor.Compose (Compose (..))
+import Data.List (sort)
+import Data.Text (Text)
+import Data.Text.Encoding (decodeUtf8)
+import Test.Tasty.Bench
+import Test.Tasty.HUnit
+import Text.Pandoc.Builder (Blocks, toList)
+import Text.Pandoc.Definition
+import Text.Pandoc.Generic (queryWith)
+import qualified Text.Pandoc.Walk as PW
+
+data PTag
+
+instance MultiTag PTag where
+  type
+    MultiTypes PTag =
+      '[ Block
+       , Inline
+       ]
+
+type DoubleList a = MatchWith [[a]] (Trav (Compose [] []) a)
+
+instance MultiSub PTag Block where
+  type
+    SubTypes PTag Block =
+      'SpecList
+        '[ ToSpec (Trav [] Inline)
+         , ToSpec (DoubleList Inline)
+         , ToSpec (Trav [] Block)
+         , ToSpec (DoubleList Block)
+         , ToSpec
+            ( Under
+                [([Inline], [[Block]])]
+                'NoSel
+                (Under ([Inline], [[Block]]) 'NoSel (Trav [] Inline))
+            )
+         , ToSpec
+            ( Under
+                [([Inline], [[Block]])]
+                'NoSel
+                (Under ([Inline], [[Block]]) 'NoSel (DoubleList Block))
+            )
+         ]
+
+instance MultiSub PTag Inline where
+  type
+    SubTypes PTag Inline =
+      'SpecList '[ToSpec (Trav [] Inline), ToSpec (Trav [] Block)]
+
+prepEnv :: IO [Block]
+prepEnv = do
+  text <- decodeUtf8 <$> B.readFile "test/text.md"
+  Right (Cm b :: Cm () Blocks) <- pure $ commonmark "test/text.md" text
+  evaluate $ force $ toList b
+
+multiLW :: [Block] -> [Text]
+multiLW = foldMap (LW.execWriter . w)
+  where
+    w = buildMultiW @PTag $ \f list ->
+      let blks x@(CodeBlock _ c) = LW.tell [c] $> x
+          blks x = f x
+          inls x@(Code _ c) = LW.tell [c] $> x
+          inls x = f x
+       in list .> blks .> inls
+
+multiSW :: [Block] -> [Text]
+multiSW = foldMap (SW.execWriter . w)
+  where
+    w = buildMultiW @PTag $ \f list ->
+      let blks x@(CodeBlock _ c) = SW.tell [c] $> x
+          blks x = f x
+          inls x@(Code _ c) = SW.tell [c] $> x
+          inls x = f x
+       in list .> blks .> inls
+
+multiCPSW :: [Block] -> [Text]
+multiCPSW = foldMap (CPSW.execWriter . w)
+  where
+    w = buildMultiW @PTag $ \f list ->
+      let blks x@(CodeBlock _ c) = CPSW.tell [c] $> x
+          blks x = f x
+          inls x@(Code _ c) = CPSW.tell [c] $> x
+          inls x = f x
+       in list .> blks .> inls
+
+multi :: [Block] -> [Text]
+multi = foldMap $
+  buildMultiQ @PTag $ \sub list ->
+    list ?> blks sub ?> inls sub
+  where
+    blks :: Query PTag [Text] -> Block -> [Text]
+    blks _ (CodeBlock _ c) = [c]
+    blks f x = f x
+    inls :: Query PTag [Text] -> Inline -> [Text]
+    inls _ (Code _ c) = [c]
+    inls f x = f x
+
+gene :: [Block] -> [Text]
+gene x = queryWith blks x <> queryWith inls x
+  where
+    blks :: Block -> [Text]
+    blks (CodeBlock _ c) = [c]
+    blks _ = []
+    inls :: Inline -> [Text]
+    inls (Code _ c) = [c]
+    inls _ = []
+
+wal :: [Block] -> [Text]
+wal x = PW.query blks x <> PW.query inls x
+  where
+    blks :: Block -> [Text]
+    blks (CodeBlock _ c) = [c]
+    blks _ = []
+    inls :: Inline -> [Text]
+    inls (Code _ c) = [c]
+    inls _ = []
+
+main :: IO ()
+main =
+  defaultMain
+    [ bgroup
+        "query"
+        [ env prepEnv $ bench "multiwalk" . nf multi
+        , env prepEnv $ bench "mw lw" . nf multiLW
+        , env prepEnv $ bench "mw sw" . nf multiSW
+        , env prepEnv $ bench "mw cpsw" . nf multiCPSW
+        , env prepEnv $ bench "syb" . nf gene
+        , env prepEnv $ bench "pandoc.walk" . nf wal
+        , env prepEnv $ \blocks ->
+            -- The other implementations return out of order (!!!) fragments, but sorted they should be the same.
+            bgroup
+              "equality"
+              [ testCase "multiwalk eq syb" (sort (multi blocks) @?= sort (gene blocks))
+              , testCase "multiwalk eq pandoc.walk" (sort (multi blocks) @?= sort (wal blocks))
+              ]
+        ]
+    ]
diff --git a/test/Main.hs b/test/Main.hs
new file mode 100644
--- /dev/null
+++ b/test/Main.hs
@@ -0,0 +1,74 @@
+module Main (main) where
+
+import Control.MultiWalk
+import Data.Functor.Compose (Compose (..))
+import Data.Functor.Identity (Identity (..))
+import GHC.Generics (Generic)
+import Test.Tasty
+import Test.Tasty.HUnit
+
+data Foo
+  = Foo1 String
+  | Foo2 String Foo
+  | Foo3 [Int] String [[Int]] Int [Int]
+  deriving (Eq, Show, Generic)
+
+data FooTag
+
+instance MultiTag FooTag where
+  type
+    MultiTypes FooTag =
+      '[ String
+       , Foo
+       , Int
+       , [Int]
+       ]
+
+instance MultiSub FooTag Foo where
+  type
+    SubTypes FooTag Foo =
+      'SpecList
+        '[ ToSpec Foo
+         , ToSpec String
+         , ToSpec (MatchWith [[Int]] (Trav (Compose [] []) Int))
+         , ToSpec [Int]
+         ]
+
+instance MultiSub FooTag String where
+  type SubTypes FooTag String = 'SpecLeaf
+
+instance MultiSub FooTag Int where
+  type SubTypes FooTag Int = 'SpecLeaf
+
+instance MultiSub FooTag [Int] where
+  type SubTypes FooTag [Int] = 'SpecList '[ToSpec (Trav [] Int)]
+
+sampleFoo :: Foo
+sampleFoo = Foo2 "bla" (Foo2 "blblo" (Foo1 "ok"))
+
+list' :: FList Identity '[String, Foo, Int, [Int]]
+list' = (Identity . ('2' :)) :.: pure :.: (Identity . (2 *)) :.: (\x -> Identity (x ++ x)) :.: FNil
+
+sampleFoo2 :: Foo
+sampleFoo2 = Foo3 [0, 1] "abc" [[4, 8, 9], [5, 6, 7]] 32 [2, 3]
+
+tests :: TestTree
+tests =
+  testGroup
+    "Tests"
+    [ testGroup
+        "Query"
+        [ testCase "Foo" $
+            query @FooTag (\(x :: Foo) -> [x]) sampleFoo
+              @?= [Foo2 "bla" (Foo2 "blblo" (Foo1 "ok")), Foo2 "blblo" (Foo1 "ok"), Foo1 "ok"]
+        , testCase "String" $
+            query @FooTag (\(x :: String) -> [x]) sampleFoo
+              @?= ["bla", "blblo", "ok"]
+        ]
+    , testCase "ModSub" $
+        walkSub @FooTag list' sampleFoo2
+          @?= Identity (Foo3 [0, 1, 0, 1] "2abc" [[8, 16, 18], [10, 12, 14]] 32 [2, 3, 2, 3])
+    ]
+
+main :: IO ()
+main = defaultMain tests