diff --git a/.github/workflows/ci_base.yml b/.github/workflows/ci_base.yml
new file mode 100644
index 0000000..a503aaa
--- /dev/null
+++ b/.github/workflows/ci_base.yml
@@ -0,0 +1,20 @@
+---
+name: CI libs/base
+
+on:
+  pull_request:
+    paths:
+      - 'dev-requirements.txt'
+      - 'pip-requirements.txt'
+      - '.github/workflows/python_reusable.yml'
+      - '.github/workflows/ci_base.yml'
+      - 'libs/base/**'
+  workflow_dispatch:  # Allows to trigger the workflow manually in GitHub UI
+
+jobs:
+  ci-libs-base:
+    uses:
+      ./.github/workflows/python_reusable.yml
+    with:
+      working-directory: libs/base
+    secrets: inherit
diff --git a/.github/workflows/ci_fancy.yml b/.github/workflows/ci_fancy.yml
new file mode 100644
index 0000000..b8cad3d
--- /dev/null
+++ b/.github/workflows/ci_fancy.yml
@@ -0,0 +1,21 @@
+---
+name: CI libs/fancy
+
+on:
+  pull_request:
+    paths:
+      - 'dev-requirements.txt'
+      - 'pip-requirements.txt'
+      - '.github/workflows/python_reusable.yml'
+      - '.github/workflows/ci_fancy.yml'
+      - 'libs/base/**'  # libs/fancy depends on libs/base
+      - 'libs/fancy/**'
+  workflow_dispatch:  # Allows to trigger the workflow manually in GitHub UI
+
+jobs:
+  ci-libs-fancy:
+    uses:
+      ./.github/workflows/python_reusable.yml
+    with:
+      working-directory: libs/fancy
+    secrets: inherit
diff --git a/.github/workflows/python_reusable.yml b/.github/workflows/python_reusable.yml
new file mode 100644
index 0000000..975d6f1
--- /dev/null
+++ b/.github/workflows/python_reusable.yml
@@ -0,0 +1,64 @@
+---
+name: Reusable Python library CI
+
+on:
+  workflow_call:
+    inputs:
+      working-directory:
+        required: true
+        type: string
+        description: "From which folder this pipeline executes"
+      install-packages:
+        description: "Space seperated list of packages to install using apt-get."
+        default: ""
+        type: string
+      # To avoid being billed 360 minutes if a step does not terminate
+      # (we've seen the setup-python step below do so!)
+      ci-timeout:
+        description: "The timeout of the ci job. Default is 25min"
+        default: 25
+        type: number
+
+jobs:
+  local-ci-py-template:
+    runs-on: ubuntu-22.04
+    timeout-minutes: 30
+
+    defaults:
+      run:
+        working-directory: ${{ inputs.working-directory }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        timeout-minutes: 5
+        with:
+          python-version-file: ${{ github.workspace }}/.python-version
+          cache: "pip"
+          cache-dependency-path: |
+            dev-requirements.txt
+            pip-requirements.txt
+            ${{ inputs.working-directory }}/requirements.txt
+
+      - name: Install extra packages
+        if: ${{ inputs.install-packages != ''}}
+        run: |
+          sudo apt-get install -y ${{ inputs.install-packages }}
+
+      - name: Install Python dependencies
+        run: |
+          pip install -r "$(git rev-parse --show-toplevel)/pip-requirements.txt"
+          pip install -r "$(git rev-parse --show-toplevel)/dev-requirements.txt" -r requirements.txt
+
+      - name: Typecheck
+        run: |
+          pyright .
+
+      - name: Test
+        run: |
+          python3 -m pytest tests/  # Assume that tests are in folder "tests/"
diff --git a/.github/workflows/top_level.yml b/.github/workflows/top_level.yml
new file mode 100644
index 0000000..e92818d
--- /dev/null
+++ b/.github/workflows/top_level.yml
@@ -0,0 +1,54 @@
+---
+name: Top-level CI
+on:
+  workflow_dispatch:
+  pull_request:
+  push:
+    branches: main  # Comment this line if you want to test the CI before opening a PR
+
+jobs:
+  ci-global:
+    runs-on: ubuntu-22.04
+    timeout-minutes: 10
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        timeout-minutes: 5
+        with:
+          python-version-file: .python-version
+          cache: 'pip'
+          cache-dependency-path: |
+            pip-requirements.txt
+            dev-requirements.txt
+
+      - name: Install Python dependencies
+        run: |
+          pip install -r pip-requirements.txt
+          pip install -r dev-requirements.txt
+
+      # in order to test the template as well, we create a temporary library in the CI
+      - name: Create library with template
+        run: |
+          cookiecutter --no-input templates/pylibrary --output-dir libs/
+          git add libs/library_name  # So that checks below apply to generated library
+
+      - name: Format Python imports
+        run: |
+          isort --check-only $(git ls-files "*.py")
+
+      - name: Format Python
+        run: |
+          black --check $(git ls-files "*.py")
+
+      - name: Lint Python
+        run: |
+          flake8 $(git ls-files "*.py")
+
+      # Are all public symbols documented? (see top-level pyproject.toml configuration)
+      - name: Lint Python doc
+        run: |
+          pylint $(git ls-files "*.py")
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..1da5487
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,164 @@
+##################################################################
+# https://github.com/github/gitignore/blob/main/Python.gitignore #
+##################################################################
+
+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
+
+# C extensions
+*.so
+
+# Distribution / packaging
+.Python
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+wheels/
+share/python-wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+MANIFEST
+
+# PyInstaller
+#  Usually these files are written by a python script from a template
+#  before PyInstaller builds the exe, so as to inject date/other infos into it.
+*.manifest
+*.spec
+
+# Installer logs
+pip-log.txt
+pip-delete-this-directory.txt
+
+# Unit test / coverage reports
+htmlcov/
+.tox/
+.nox/
+.coverage
+.coverage.*
+.cache
+nosetests.xml
+coverage.xml
+*.cover
+*.py,cover
+.hypothesis/
+.pytest_cache/
+cover/
+
+# Translations
+*.mo
+*.pot
+
+# Django stuff:
+*.log
+local_settings.py
+db.sqlite3
+db.sqlite3-journal
+
+# Flask stuff:
+instance/
+.webassets-cache
+
+# Scrapy stuff:
+.scrapy
+
+# Sphinx documentation
+docs/_build/
+
+# PyBuilder
+.pybuilder/
+target/
+
+# Jupyter Notebook
+.ipynb_checkpoints
+
+# IPython
+profile_default/
+ipython_config.py
+
+# pyenv
+#   For a library or package, you might want to ignore these files since the code is
+#   intended to run in multiple environments; otherwise, check them in:
+# .python-version
+
+# pipenv
+#   According to pypa/pipenv#598, it is recommended to include Pipfile.lock in version control.
+#   However, in case of collaboration, if having platform-specific dependencies or dependencies
+#   having no cross-platform support, pipenv may install dependencies that don't work, or not
+#   install all needed dependencies.
+#Pipfile.lock
+
+# poetry
+#   Similar to Pipfile.lock, it is generally recommended to include poetry.lock in version control.
+#   This is especially recommended for binary packages to ensure reproducibility, and is more
+#   commonly ignored for libraries.
+#   https://python-poetry.org/docs/basic-usage/#commit-your-poetrylock-file-to-version-control
+#poetry.lock
+
+# pdm
+#   Similar to Pipfile.lock, it is generally recommended to include pdm.lock in version control.
+#pdm.lock
+#   pdm stores project-wide configurations in .pdm.toml, but it is recommended to not include it
+#   in version control.
+#   https://pdm.fming.dev/#use-with-ide
+.pdm.toml
+
+# PEP 582; used by e.g. github.com/David-OConnor/pyflow and github.com/pdm-project/pdm
+__pypackages__/
+
+# Celery stuff
+celerybeat-schedule
+celerybeat.pid
+
+# SageMath parsed files
+*.sage.py
+
+# Environments
+.env
+.venv
+env/
+venv/
+ENV/
+env.bak/
+venv.bak/
+
+# Spyder project settings
+.spyderproject
+.spyproject
+
+# Rope project settings
+.ropeproject
+
+# mkdocs documentation
+/site
+
+# mypy
+.mypy_cache/
+.dmypy.json
+dmypy.json
+
+# Pyre type checker
+.pyre/
+
+# pytype static type analyzer
+.pytype/
+
+# Cython debug symbols
+cython_debug/
+
+# PyCharm
+#  JetBrains specific template is maintained in a separate JetBrains.gitignore that can
+#  be found at https://github.com/github/gitignore/blob/main/Global/JetBrains.gitignore
+#  and can be added to the global gitignore or merged into this file.  For a more nuclear
+#  option (not recommended) you can uncomment the following to ignore the entire idea folder.
+#.idea/
diff --git a/.mergify.yml b/.mergify.yml
new file mode 100644
index 0000000..58c1672
--- /dev/null
+++ b/.mergify.yml
@@ -0,0 +1,28 @@
+queue_rules:
+  - name: default
+
+pull_request_rules:
+  - name: automatic merge
+    conditions:
+      - "#approved-reviews-by>=1"
+      - "label=merge-queue"
+      - "base=main"
+    actions:
+      queue:
+        name: default
+        method: rebase
+
+  - name: delete head branch after merge
+    conditions:
+      - merged
+      - closed
+    actions:
+      delete_head_branch: {}
+
+  - name: remove from merge-queue after merge
+    conditions:
+      - merged
+    actions:
+      label:
+        remove:
+          - "merge-queue"
diff --git a/.python-version b/.python-version
new file mode 100644
index 0000000..78c9a28
--- /dev/null
+++ b/.python-version
@@ -0,0 +1 @@
+3.9.12
diff --git a/CODEOWNERS b/CODEOWNERS
new file mode 100644
index 0000000..efc3a71
--- /dev/null
+++ b/CODEOWNERS
@@ -0,0 +1,58 @@
+# CODEOWNERS
+
+# This file declares which users are qualified to review and approve changes to
+# specific files or paths in this repository. It helps contributors to identify
+# whom to consult for assistance on merge requests.
+
+# To understand the syntax and semantics of this file, see:
+#   https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
+#
+# Note that rules defined later in the file take precedence over the rules
+# defined earlier.
+
+# The owners in this rule will be the default owners for everything in
+# the repo (if no other rule below matches the path of a modified file)
+*                       @ribeirojose @mfw
+
+# Global files. Review load should be spread among engineers,
+# to avoid burying one person under review duties
+.gitignore              @ribeirojose @mfw
+.mergify.yml            @ribeirojose @mfw
+
+# Python tooling. In a real setup, we recommend to create a GitHub team
+# (https://docs.github.com/en/organizations/organizing-members-into-teams/about-teams)
+# in charge of Python tooling. So the owner would be @your_org/python_team
+.python-version         @ribeirojose @mfw
+dev-requirements.txt    @ribeirojose @mfw
+pip-requirements.txt    @ribeirojose @mfw
+pyproject.toml          @ribeirojose @mfw
+tox.ini                 @ribeirojose @mfw
+
+# CI. In a real setup, we recommend to create a GitHub team
+# (https://docs.github.com/en/organizations/organizing-members-into-teams/about-teams)
+# in charge of CI/CD. So the owner would be @your_org/cicd_team
+# Note that the leading slash makes the rule include all nested subdirectories
+/.github/workflows      @ribeirojose @mfw
+
+# Libraries. The owner of a library is usually its first author, naturally
+# spreading review load.
+/libs/base              @ribeirojose @mfw
+/libs/fancy             @ribeirojose @mfw
+
+# Scripts
+/scripts                @ribeirojose @mfw
+
+# Templates
+/templates              @ribeirojose @mfw
+
+# CODEOWNERS Owners
+#
+# These users are authorized to change authorizations.
+#
+# Note that this stanza MUST appear LAST so that it takes precedence over prior rules.
+# This should be unnecessary if rules above are correct, but being safe.
+#
+# In a real setup, we recommend to create a GitHub team
+# (https://docs.github.com/en/organizations/organizing-members-into-teams/about-teams)
+# to group the administrators. So the owner would be @your_org/admins
+/CODEOWNERS             @ribeirojose @mfw
diff --git a/README.md b/README.md
index 0c3f06a..ad9aed6 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,39 @@
 # cow-py
+
 CoW Protocol Python SDK
 
 [Help the herd 🐮](https://snapshot.org/#/cowgrants.eth/proposal/0x29bde0a0789a15f2255e11bdff088b4ffdf491729250dbe93b8b0776beb7f999)
+
+
+## templated from python-monorepo-example
+
+This repository is a template for a Python monorepo with projects-specific
+virtual environments. It uses [Pip](https://pypi.org/project/pip/) for installing dependencies.
+
+Because this repository is a [GitHub template](https://docs.github.com/en/repositories/creating-and-managing-repositories/creating-a-template-repository),
+you can duplicate it by using
+[_Creating a repository from a template_](https://docs.github.com/en/repositories/creating-and-managing-repositories/creating-a-repository-from-a-template) in the GitHub UI. It will create
+a copy of this repository that is not a fork.
+
+This repository's design is explained on the Tweag blog:
+
+* [Python monorepo; part 1](https://www.tweag.io/blog/2023-04-04-python-monorepo-1/)
+  describes the monorepo's structure, how libraries are linked together and which
+  tools are used.
+* [Python monorepo; part 2](https://www.tweag.io/blog/2023-07-13-python-monorepo-2)
+  describes the monorepo's CI, striking a good balance between being easy to use and being
+  featureful.
+
+The design strives to be simple, to work well in a startup environment where
+CI specialists are not yet available, and yet to achieve a great deal
+of [reproducibility](https://reproducible-builds.org/) to prepare for scaling.
+
+We use a virtual environment per library/project, to allow dependencies to
+diverge if you need to. Another alternative is to have a single virtual environment
+for the entire repository, to maximize uniformity. Choose what suits you best.
+
+We use [editable installs](https://setuptools.pypa.io/en/latest/userguide/development_mode.html)
+for dependencies _within_ this repository, so that changes to a library are reflected
+immediately in code depending on the said library. This implements
+the _live at HEAD_ workflow, a term made popular by
+[Titus Winters](https://www.youtube.com/watch?v=tISy7EJQPzI) from Google.
diff --git a/dev-requirements.txt b/dev-requirements.txt
new file mode 100644
index 0000000..063464c
--- /dev/null
+++ b/dev-requirements.txt
@@ -0,0 +1,7 @@
+black==23.1.0
+cookiecutter==2.1.1
+flake8==6.0.0
+isort==5.12.0
+pylint==2.16.3
+pyright==1.1.296
+pytest==7.2.1
diff --git a/libs/base/.tool-versions b/libs/base/.tool-versions
new file mode 100644
index 0000000..926567c
--- /dev/null
+++ b/libs/base/.tool-versions
@@ -0,0 +1 @@
+python 3.9.12
diff --git a/libs/base/README.md b/libs/base/README.md
new file mode 100644
index 0000000..c7e4193
--- /dev/null
+++ b/libs/base/README.md
@@ -0,0 +1,42 @@
+# base
+
+A Python package by cow_py.
+
+The project owner is [@smelc](https://github.com/smelc).
+
+## Development
+
+If not already in a virtual environement, create and use one.
+Read about it in the Python documentation: [venv — Creation of virtual environments](https://docs.python.org/3/library/venv.html).
+
+```
+python3 -m venv .venv
+source .venv/bin/activate
+```
+
+Install the pinned pip version:
+
+```
+pip install -r $(git rev-parse --show-toplevel)/pip-requirements.txt
+```
+
+Finally, install the dependencies:
+
+```
+pip install -r $(git rev-parse --show-toplevel)/dev-requirements.txt -r requirements.txt
+```
+
+## Testing
+
+Execute tests from the library's folder (after having loaded the virtual environment,
+see above) as follows:
+
+```
+python3 -m pytest tests/
+```
+
+Execute the library's CI locally with [act](https://github.com/nektos/act) as follows:
+
+```
+act -j ci-libs-fancy
+```
diff --git a/libs/base/cow_py/base/__init__.py b/libs/base/cow_py/base/__init__.py
new file mode 100644
index 0000000..fabe685
--- /dev/null
+++ b/libs/base/cow_py/base/__init__.py
@@ -0,0 +1,3 @@
+from cow_py.base import adder2
+
+__all__ = ["adder2"]
diff --git a/libs/base/cow_py/base/adder2.py b/libs/base/cow_py/base/adder2.py
new file mode 100644
index 0000000..01a277f
--- /dev/null
+++ b/libs/base/cow_py/base/adder2.py
@@ -0,0 +1,14 @@
+"""Example module of the base library"""
+
+
+def add2(x: int, y: int) -> int:
+    """
+    Adds two integers
+
+    Args:
+        x: the left operand
+        y: the right operand
+    Returns:
+        The sum of x and y
+    """
+    return x + y
diff --git a/libs/base/pyproject.toml b/libs/base/pyproject.toml
new file mode 100644
index 0000000..67d34cc
--- /dev/null
+++ b/libs/base/pyproject.toml
@@ -0,0 +1,36 @@
+[build-system]
+requires = ["poetry-core>=1.0.0"]
+build-backend = "poetry.core.masonry.api"
+
+[tool.poetry]
+name = "cow_py-base"
+version = "0.0.1"
+description = "Shared library"
+authors = [ "Clément Hurlin <clement.hurlin@tweag.io>" ]
+packages = [
+  { include = "cow_py" }
+]
+
+[tool.poetry.dependencies]
+python = ">=3.9"
+numpy = ">=1.21"
+
+[tool.poetry.dev-dependencies]
+black = "^23.1.0"
+flake8 = "^6.0.0"
+isort = "^5.12.0"
+pylint = "^2.16.3"
+pyright = "^1.1.296"
+pytest = "^7.2.1"
+
+[tool.black]
+line-length = 100
+target-version = ['py39']
+
+[tool.pyright]
+
+[tool.pylint."messages control"]
+disable = "all"
+enable = ["empty-docstring", "missing-module-docstring", "missing-class-docstring", "missing-function-docstring"]
+ignore = ["setup.py", "__init__.py"]
+ignore-paths = ["tests"]
diff --git a/libs/base/requirements.txt b/libs/base/requirements.txt
new file mode 100644
index 0000000..cfe20ba
--- /dev/null
+++ b/libs/base/requirements.txt
@@ -0,0 +1,4 @@
+# Add pinned dependencies to install for development
+# Development dependencies (black, flake8, etc.) are in the top-level dev-requirements.txt file
+
+numpy==1.22.0  # Actually unused, for demonstration only
diff --git a/libs/base/setup.cfg b/libs/base/setup.cfg
new file mode 100644
index 0000000..1192d74
--- /dev/null
+++ b/libs/base/setup.cfg
@@ -0,0 +1 @@
+# This file is empty, but is needed by setuptools to detect pyproject.toml (for now)
diff --git a/libs/base/tests/conftest.py b/libs/base/tests/conftest.py
new file mode 100644
index 0000000..11afce2
--- /dev/null
+++ b/libs/base/tests/conftest.py
@@ -0,0 +1,13 @@
+"""
+Shared configuration and fixtures for pytest.
+"""
+
+import pytest
+
+
+@pytest.fixture
+def some_text() -> str:
+    """
+    Some fixture which can be reused in all test modules.
+    """
+    return "Some text"
diff --git a/libs/base/tests/test_base.py b/libs/base/tests/test_base.py
new file mode 100644
index 0000000..9d2c3a7
--- /dev/null
+++ b/libs/base/tests/test_base.py
@@ -0,0 +1,24 @@
+"""
+Tests of adder2
+"""
+
+from cow_py import base
+
+
+def test_zero() -> None:
+    """Tests the neutral element"""
+    assert base.adder2.add2(0, 0) == 0
+    assert base.adder2.add2(0, 3) == 3
+    assert base.adder2.add2(0, 3) == base.adder2.add2(3, 0)
+
+
+def test_some() -> None:
+    """Some unit tests of add2"""
+    assert base.adder2.add2(1, 3) == 4
+    assert base.adder2.add2(1, 3) == base.adder2.add2(3, 1)
+    assert base.adder2.add2(100, 2) == (base.adder2.add2(100, 1) + base.adder2.add2(1, 0))
+
+
+# In a true repository we would add property tests here,
+# using hypothesis:
+# https://hypothesis.readthedocs.io/en/latest/
diff --git a/libs/fancy/README.md b/libs/fancy/README.md
new file mode 100644
index 0000000..b3a7626
--- /dev/null
+++ b/libs/fancy/README.md
@@ -0,0 +1,42 @@
+# fancy
+
+A Python package by cow_py.
+
+The project owner is our gentleman [@GuillaumeDesforges](https://github.com/GuillaumeDesforges).
+
+## Development
+
+If not already in a virtual environement, create and use one.
+Read about it in the Python documentation: [venv — Creation of virtual environments](https://docs.python.org/3/library/venv.html).
+
+```
+python3 -m venv .venv
+source .venv/bin/activate
+```
+
+Install the pinned pip version:
+
+```
+pip install -r $(git rev-parse --show-toplevel)/pip-requirements.txt
+```
+
+Finally, install the dependencies:
+
+```
+pip install -r $(git rev-parse --show-toplevel)/dev-requirements.txt -r requirements.txt
+```
+
+## Testing
+
+Execute tests from the library's folder (after having loaded the virtual environment,
+see above) as follows:
+
+```
+python3 -m pytest tests/
+```
+
+Execute the library's CI locally with [act](https://github.com/nektos/act) as follows:
+
+```
+act -j ci-libs-fancy
+```
diff --git a/libs/fancy/cow_py/fancy/__init__.py b/libs/fancy/cow_py/fancy/__init__.py
new file mode 100644
index 0000000..853cb41
--- /dev/null
+++ b/libs/fancy/cow_py/fancy/__init__.py
@@ -0,0 +1,3 @@
+from cow_py.fancy import adder3
+
+__all__ = ["adder3"]
diff --git a/libs/fancy/cow_py/fancy/adder3.py b/libs/fancy/cow_py/fancy/adder3.py
new file mode 100644
index 0000000..8bb9db2
--- /dev/null
+++ b/libs/fancy/cow_py/fancy/adder3.py
@@ -0,0 +1,17 @@
+"""Example module of the fancy library, consuming the base library"""
+
+from cow_py import base
+
+
+def add3(x: int, y: int, z: int) -> int:
+    """
+    Adds three integers
+
+    Args:
+        x: the left operand
+        y: the middle operand
+        z: the right operand
+    Returns:
+        The sum of x, y, and z
+    """
+    return base.adder2.add2(base.adder2.add2(x, y), z)
diff --git a/libs/fancy/pyproject.toml b/libs/fancy/pyproject.toml
new file mode 100644
index 0000000..604fe8c
--- /dev/null
+++ b/libs/fancy/pyproject.toml
@@ -0,0 +1,37 @@
+[build-system]
+requires = ["poetry-core>=1.0.0"]
+build-backend = "poetry.core.masonry.api"
+
+[tool.poetry]
+name = "cow_py-fancy"
+version = "0.0.1"
+description = "High-level library"
+authors = [ "Clément Hurlin <clement.hurlin@tweag.io>" ]
+packages = [
+  { include = "cow_py" }
+]
+
+[tool.poetry.dependencies]
+python = ">=3.9"
+numpy = ">=1.21"
+cow_py-base = "*"
+
+[tool.poetry.dev-dependencies]
+black = "^23.1.0"
+flake8 = "^6.0.0"
+isort = "^5.12.0"
+pylint = "^2.16.3"
+pyright = "^1.1.296"
+pytest = "^7.2.1"
+
+[tool.black]
+line-length = 100
+target-version = ['py39']
+
+[tool.pyright]
+
+[tool.pylint."messages control"]
+disable = "all"
+enable = ["empty-docstring", "missing-module-docstring", "missing-class-docstring", "missing-function-docstring"]
+ignore = ["setup.py", "__init__.py"]
+ignore-paths = ["tests"]
diff --git a/libs/fancy/requirements.txt b/libs/fancy/requirements.txt
new file mode 100644
index 0000000..1453fe8
--- /dev/null
+++ b/libs/fancy/requirements.txt
@@ -0,0 +1,4 @@
+# Add pinned dependencies to install for development
+# Development dependencies (black, flake8, etc.) are in the top-level dev-requirements.txt file
+
+-e ../base
diff --git a/libs/fancy/setup.cfg b/libs/fancy/setup.cfg
new file mode 100644
index 0000000..1192d74
--- /dev/null
+++ b/libs/fancy/setup.cfg
@@ -0,0 +1 @@
+# This file is empty, but is needed by setuptools to detect pyproject.toml (for now)
diff --git a/libs/fancy/tests/conftest.py b/libs/fancy/tests/conftest.py
new file mode 100644
index 0000000..11afce2
--- /dev/null
+++ b/libs/fancy/tests/conftest.py
@@ -0,0 +1,13 @@
+"""
+Shared configuration and fixtures for pytest.
+"""
+
+import pytest
+
+
+@pytest.fixture
+def some_text() -> str:
+    """
+    Some fixture which can be reused in all test modules.
+    """
+    return "Some text"
diff --git a/libs/fancy/tests/test_fancy.py b/libs/fancy/tests/test_fancy.py
new file mode 100644
index 0000000..87367ef
--- /dev/null
+++ b/libs/fancy/tests/test_fancy.py
@@ -0,0 +1,34 @@
+"""
+Tests of adder3
+"""
+
+# from cow_py import base, fancy # don't! This makes pyright not able to link
+# one of the two packages (is it https://github.com/microsoft/pyright/issues/2882?)
+from cow_py import base
+from cow_py import fancy
+
+
+def test_zero() -> None:
+    """Test of add3 with zero"""
+    assert fancy.adder3.add3(0, 0, 0) == 0
+    assert fancy.adder3.add3(0, 1, 0) == 1
+    assert fancy.adder3.add3(0, 1, 2) == 3
+    assert fancy.adder3.add3(1, 2, 0) == 3
+
+
+def test_add2_add3() -> None:
+    """Test relation between add2 and add3"""
+    assert fancy.adder3.add3(1, 2, 3) == (base.adder2.add2(1, 2) + base.adder2.add2(0, 3))
+    assert fancy.adder3.add3(1, 2, 0) == base.adder2.add2(1, 2)
+
+
+def test_some() -> None:
+    """Some unit tests of add3"""
+    assert fancy.adder3.add3(1, 2, 3) == 6
+    assert fancy.adder3.add3(1, 2, 3) == fancy.adder3.add3(3, 2, 1)
+    assert fancy.adder3.add3(100, 2, 3) == 105
+
+
+# In a true repository we would add property tests here,
+# using hypothesis:
+# https://hypothesis.readthedocs.io/en/latest/
diff --git a/pip-requirements.txt b/pip-requirements.txt
new file mode 100644
index 0000000..6c7e07f
--- /dev/null
+++ b/pip-requirements.txt
@@ -0,0 +1,2 @@
+# We pin the version of pip, so that all commands use the same version
+pip==24.0
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..595b6de
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,36 @@
+# Technically this section is not required, because this top-level pyproject.ml
+# is only used for configuring tools, not for building anything.
+# But we nevertheless have it, to be consistent with projects and libraries.
+[build-system]
+requires = ["poetry-core>=1.0.0"]
+build-backend = "poetry.core.masonry.api"
+
+[tool.poetry]
+
+[tool.poetry.dev-dependencies]
+black = "^23.1.0"
+flake8 = "^6.0.0"
+isort = "^5.12.0"
+pyright = "^1.1.296"
+pytest = "^7.2.1"
+
+[tool.black]
+line-length = 100
+target-version = ['py39']
+force-exclude = '''
+^/archives/.*$
+'''
+
+[tool.pylint."messages control"]
+disable = "all"
+enable = ["empty-docstring", "missing-module-docstring", "missing-class-docstring", "missing-function-docstring"]
+ignore = ["setup.py", "__init__.py"]
+ignore-paths = ['.*/tests', ]
+
+[tool.isort]
+profile = "black"
+line_length = 100
+auto_identify_namespace_packages = false
+force_single_line = true # pyright doesn't like implicit namespace + single line (related to https://github.com/microsoft/pyright/issues/2882?)
+known_first_party = ["cow_py"]
+extend_skip = ["archives"]
diff --git a/scripts/rebuild-sandbox.sh b/scripts/rebuild-sandbox.sh
new file mode 100755
index 0000000..ea079c7
--- /dev/null
+++ b/scripts/rebuild-sandbox.sh
@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+#
+# Builds (or rebuilds) the sandbox of a library. Run as:
+#
+# ./$(git rev-parse --show-toplevel)/scripts/rebuilds-sandbox.sh
+#
+# in any library directory. You likely want to source the sandbox in your
+# shell after having called this script. Do it with:
+#
+# source .venv/bin/activate
+
+set -x
+
+rm -Rf ".venv"
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r $(git rev-parse --show-toplevel)/pip-requirements.txt
+pip install -r $(git rev-parse --show-toplevel)/dev-requirements.txt -r requirements.txt
diff --git a/tox.ini b/tox.ini
new file mode 100644
index 0000000..bf6ea39
--- /dev/null
+++ b/tox.ini
@@ -0,0 +1,9 @@
+# flake8 doesn't support configuration through pyproject.toml, hence this file
+[flake8]
+max-line-length = 100
+# required for compatibility with black
+extend-ignore = N812, E203
+# list names frequently used for sandboxes
+exclude =
+    .venv
+    venv