diff --git a/nix/readtree.nix b/nix/readtree.nix
deleted file mode 100644
index 066d326..0000000
--- a/nix/readtree.nix
+++ /dev/null
@@ -1,96 +0,0 @@
-# The MIT License (MIT)
-# 
-# Copyright (c) 2019 Vincent Ambo
-# 
-# Permission is hereby granted, free of charge, to any person obtaining a copy
-# of this software and associated documentation files (the "Software"), to deal
-# in the Software without restriction, including without limitation the rights
-# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-# copies of the Software, and to permit persons to whom the Software is
-# furnished to do so, subject to the following conditions:
-# 
-# The above copyright notice and this permission notice shall be included in all
-# copies or substantial portions of the Software.
-# 
-# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-# SOFTWARE.
-
-{ ... }:
-
-args: initPath:
-
-let
-  inherit (builtins)
-    attrNames
-    baseNameOf
-    filter
-    hasAttr
-    head
-    isAttrs
-    length
-    listToAttrs
-    map
-    match
-    readDir
-    substring;
-
-  argsWithPath = parts:
-    let meta.locatedAt = parts;
-    in meta // (if isAttrs args then args else args meta);
-
-  readDirVisible = path:
-    let
-      children = readDir path;
-      isVisible = f: f == ".skip-subtree" || (substring 0 1 f) != ".";
-      names = filter isVisible (attrNames children);
-    in listToAttrs (map (name: {
-      inherit name;
-      value = children.${name};
-    }) names);
-
-  # The marker is added to every set that was imported directly by
-  # readTree.
-  importWithMark = path: parts:
-    let imported = import path (argsWithPath parts);
-    in if (isAttrs imported)
-      then imported // { __readTree = true; }
-      else imported;
-
-  nixFileName = file:
-    let res = match "(.*)\.nix" file;
-    in if res == null then null else head res;
-
-  readTree = path: parts:
-    let
-      dir = readDirVisible path;
-      self = importWithMark path parts;
-      joinChild = c: path + ("/" + c);
-
-      # Import subdirectories of the current one, unless the special
-      # `.skip-subtree` file exists which makes readTree ignore the
-      # children.
-      #
-      # This file can optionally contain information on why the tree
-      # should be ignored, but its content is not inspected by
-      # readTree
-      filterDir = f: dir."${f}" == "directory";
-      children = if hasAttr ".skip-subtree" dir then [] else map (c: {
-        name = c;
-        value = readTree (joinChild c) (parts ++ [ c ]);
-      }) (filter filterDir (attrNames dir));
-
-      # Import Nix files
-      nixFiles = filter (f: f != null) (map nixFileName (attrNames dir));
-      nixChildren = map (c: let p = joinChild (c + ".nix"); in {
-        name = c;
-        value = importWithMark p (parts ++ [ c ]);
-      }) nixFiles;
-    in if dir ? "default.nix"
-      then (if isAttrs self then self // (listToAttrs children) else self)
-      else listToAttrs (nixChildren ++ children);
-in readTree initPath [ (baseNameOf initPath) ]
diff --git a/nix/readtree/LICENSE b/nix/readtree/LICENSE
new file mode 100644
index 0000000..bdc72a2
--- /dev/null
+++ b/nix/readtree/LICENSE
@@ -0,0 +1,22 @@
+The MIT License (MIT)
+
+Copyright (c) 2019 Vincent Ambo
+Copyright (c) 2020-2021 The TVL Authors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/nix/readtree/README.md b/nix/readtree/README.md
new file mode 100644
index 0000000..138abbe
--- /dev/null
+++ b/nix/readtree/README.md
@@ -0,0 +1,84 @@
+readTree
+========
+
+This is a Nix program that builds up an attribute set tree for a large
+repository based on the filesystem layout.
+
+It is in fact the tool that lays out the attribute set of this repository.
+
+As an example, consider a root (`.`) of a repository and a layout such as:
+
+```
+.
+├── third_party
+│   ├── default.nix
+│   └── rustpkgs
+│       ├── aho-corasick.nix
+│       └── serde.nix
+└── tools
+    ├── cheddar
+    │   └── default.nix
+    └── roquefort.nix
+```
+
+When `readTree` is called on that tree, it will construct an attribute set with
+this shape:
+
+```nix
+{
+    tools = {
+        cheddar = ...;
+        roquefort = ...;
+    };
+
+    third_party = {
+        # the `default.nix` of this folder might have had arbitrary other
+        # attributes here, such as this:
+        favouriteColour = "orange";
+
+        rustpkgs = {
+            aho-corasick = ...;
+            serde = ...;
+        };
+    };
+}
+```
+
+Every imported Nix file that yields an attribute set will have a `__readTree =
+true;` attribute merged into it.
+
+## Traversal logic
+
+`readTree` will follow any subdirectories of a tree and import all Nix files,
+with some exceptions:
+
+* A folder can declare that its children are off-limit by containing a
+  `.skip-subtree` file. Since the content of the file is not checked, it can be
+  useful to leave a note for a human in the file.
+* If a folder contains a `default.nix` file, no *sibling* Nix files will be
+  imported - however children are traversed as normal.
+* If a folder contains a `default.nix` it is loaded and, if it evaluates to a
+  set, *merged* with the children. If it evaluates to anything else the children
+  are *not traversed*.
+* The `default.nix` of the top-level folder on which readTree is
+  called is **not** read to avoid infinite recursion (as, presumably,
+  this file is where readTree itself is called).
+
+Traversal is lazy, `readTree` will only build up the tree as requested. This
+currently has the downside that directories with no importable files end up in
+the tree as empty nodes (`{}`).
+
+## Import structure
+
+`readTree` is called with two parameters: The arguments to pass to all imports,
+and the initial path at which to start the traversal.
+
+The package headers in this repository follow the form `{ pkgs, ... }:` where
+`pkgs` is a fixed-point of the entire package tree (see the `default.nix` at the
+root of the depot).
+
+In theory `readTree` can pass arguments of different shapes, but I have found
+this to be a good solution for the most part.
+
+Note that `readTree` does not currently make functions overridable, though it is
+feasible that it could do that in the future.
diff --git a/nix/readtree/default.nix b/nix/readtree/default.nix
new file mode 100644
index 0000000..633915f
--- /dev/null
+++ b/nix/readtree/default.nix
@@ -0,0 +1,116 @@
+# Copyright (c) 2019 Vincent Ambo
+# Copyright (c) 2020-2021 The TVL Authors
+# SPDX-License-Identifier: MIT
+#
+# Provides a function to automatically read a a filesystem structure
+# into a Nix attribute set.
+#
+# Optionally accepts an argument `argsFilter` on import, which is a
+# function that receives the current tree location (as a list of
+# strings) and the argument set and can arbitrarily modify it.
+{ argsFilter ? (x: _parts: x)
+, ... }:
+
+let
+  inherit (builtins)
+    attrNames
+    baseNameOf
+    concatStringsSep
+    filter
+    hasAttr
+    head
+    isAttrs
+    length
+    listToAttrs
+    map
+    match
+    readDir
+    substring;
+
+  assertMsg = pred: msg:
+    if pred
+    then true
+    else builtins.trace msg false;
+
+  argsWithPath = args: parts:
+    let meta.locatedAt = parts;
+    in meta // (if isAttrs args then args else args meta);
+
+  readDirVisible = path:
+    let
+      children = readDir path;
+      isVisible = f: f == ".skip-subtree" || (substring 0 1 f) != ".";
+      names = filter isVisible (attrNames children);
+    in listToAttrs (map (name: {
+      inherit name;
+      value = children.${name};
+    }) names);
+
+  # Create a mark containing the location of this attribute.
+  marker = parts: {
+    __readTree = parts;
+  };
+
+  # The marker is added to every set that was imported directly by
+  # readTree.
+  importWithMark = args: path: parts:
+    let
+      importedFile = import path;
+      pathType = builtins.typeOf importedFile;
+      imported =
+        assert assertMsg
+          (pathType == "lambda")
+          "readTree: trying to import ${toString path}, but it’s a ${pathType}, you need to make it a function like { depot, pkgs, ... }";
+        importedFile (argsFilter (argsWithPath args parts) parts);
+    in if (isAttrs imported)
+      then imported // (marker parts)
+      else imported;
+
+  nixFileName = file:
+    let res = match "(.*)\\.nix" file;
+    in if res == null then null else head res;
+
+  readTree = { args, initPath, rootDir, parts }:
+    let
+      dir = readDirVisible initPath;
+      joinChild = c: initPath + ("/" + c);
+
+      self = if rootDir
+        then { __readTree = []; }
+        else importWithMark args initPath parts;
+
+      # Import subdirectories of the current one, unless the special
+      # `.skip-subtree` file exists which makes readTree ignore the
+      # children.
+      #
+      # This file can optionally contain information on why the tree
+      # should be ignored, but its content is not inspected by
+      # readTree
+      filterDir = f: dir."${f}" == "directory";
+      children = if hasAttr ".skip-subtree" dir then [] else map (c: {
+        name = c;
+        value = readTree {
+          args = args;
+          initPath = (joinChild c);
+          rootDir = false;
+          parts = (parts ++ [ c ]);
+        };
+      }) (filter filterDir (attrNames dir));
+
+      # Import Nix files
+      nixFiles = filter (f: f != null) (map nixFileName (attrNames dir));
+      nixChildren = map (c: let p = joinChild (c + ".nix"); in {
+        name = c;
+        value = importWithMark args p (parts ++ [ c ]);
+      }) nixFiles;
+    in if dir ? "default.nix"
+      then (if isAttrs self then self // (listToAttrs children) else self)
+      else (listToAttrs (nixChildren ++ children) // (marker parts));
+
+in {
+  __functor = _: args: initPath: readTree {
+    inherit args initPath;
+    rootDir = true;
+    parts = [];
+  };
+}