diff --git a/.drone.yml b/.drone.yml
index f7cc74d..0c07a22 100644
--- a/.drone.yml
+++ b/.drone.yml
@@ -7,4 +7,5 @@ steps:
   - name: python-tests
     image: themattrix/tox
     commands:
-      - tox
+      # python 3.9 is not included in themattrix/tox yet
+      - tox -e py36,py37,py38,py36-lint,py37-lint,py38-lint
diff --git a/README.md b/README.md
index 7426900..32d1568 100644
--- a/README.md
+++ b/README.md
@@ -1,13 +1,16 @@
 # Simplex dictionary
 
-[![Python Versions](https://img.shields.io/badge/python-3.6%20|%203.7-blue.svg)](https://img.shields.io/badge/python-3.6%20|%203.7-blue.svg)
+[![Python Versions](https://img.shields.io/badge/python-3.6%20|%203.7-blue.svg)](https://img.shields.io/badge/python-3.6%20|%203.7%20|%203.8%20|%203.9-blue.svg)
 [![License](https://img.shields.io/badge/license-MIT-green.svg)](https://img.shields.io/badge/license-MIT-green.svg)
 [![Build Status](https://drone.typename.fr/api/badges/mikael.capelle/simplex/status.svg)](https://drone.typename.fr/mikael.capelle/simplex)
 
-`simplex` is a small python module that implements a `simplex_dictionary` structure representing
-a dictionary for the [Simplex algorithm](https://en.wikipedia.org/wiki/Simplex_algorithm).
+`simplex` is a small python module that implements a `simplex_dictionary` structure
+representing a dictionary for the
+[Simplex algorithm](https://en.wikipedia.org/wiki/Simplex_algorithm).
+The package is intended for educational purpose, e.g., to have students implement the
+Simplex algorithm based on the structure.
 
-# Basic usage
+## Basic usage
 
 The following code creates a simplex dictionary with 5 variables:
 
@@ -38,10 +41,15 @@ sdict.z = 0
 # Coefficients of the non-basic variables in the objective function:
 sdict.c[x1] = 3
 sdict.c[x2] = 2
+
+# Trying to set the coefficients for a basic variables that should be 0 raise an
+# exception:
+sdict.c[x3] = 4  # raises "x_3 is not a non-basic variable."
+sdict.b[x1] = 1  # raises "x_1 is not a basic variable."
 ```
 
-When using the `simplex` module within a jupyter notebook, the `display()` method outputs
-a latex version of the dictionary:
+When using the `simplex` module within a jupyter notebook, the `display()` method
+outputs a latex version of the dictionary:
 
 ```python
 sdict.display(name='S_0')
diff --git a/setup.cfg b/setup.cfg
index 36ee50d..3e9f8d8 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -9,14 +9,11 @@ extend-ignore =
 warn_return_any = True
 warn_unused_configs = True
 
-[mypy-pytest.*]
-ignore_missing_imports = True
-
 [mypy-IPython.*]
 ignore_missing_imports = True
 
 [tox:tox]
-envlist = py{36,37,38},py{36,37,38}-lint
+envlist = py{36,37,38,39},py{36,37,38,39}-lint
 
 [testenv]
 deps =
@@ -24,13 +21,14 @@ deps =
 commands =
     pytest tests
 
-[testenv:py{36,37,38}-lint]
+[testenv:py{36,37,38,39}-lint]
 deps =
     black
     flake8
     flake8-black
     mypy
     isort
+
 commands =
     black --check --diff setup.py simplex tests
     flake8 simplex tests
diff --git a/setup.py b/setup.py
index 4e1e7a6..066748a 100644
--- a/setup.py
+++ b/setup.py
@@ -8,7 +8,7 @@ with open("README.md", "r") as fh:
 test_requires = ["pytest", "mypy", "black", "flake8", "flake8-black"]
 
 setuptools.setup(
-    name="simplex",  # Replace with your own username
+    name="simplex-dictionary",
     version="0.0.1",
     author="Mikaël Capelle",
     author_email="capelle.mikael@gmail.com",
diff --git a/simplex/__init__.py b/simplex/__init__.py
index 9fdac14..677ff21 100644
--- a/simplex/__init__.py
+++ b/simplex/__init__.py
@@ -1,4 +1,3 @@
 # -*- encoding: utf-8 -*-
 
-
 from .simplex_dictionary import simplex_dictionary  # noqa: F401
diff --git a/simplex/py.typed b/simplex/py.typed
new file mode 100644
index 0000000..e69de29