* Extend the specification with a new restriction: overloaded names
  and let-bound names live in two different spaces.

* sujet.pdf:
  - Alleviate the tasks.
  - Authorize the project to be done by a team of two students
    (but with extra tasks)
* Update sujet.pdf. A wrong version was pushed earlier.
* Fix typos.
Merge remote-tracking branch 'upstream/master'