Fix #43 : placeholders conflict in fixtures composition

[Xephi]

Proposition fix for #43

[jeantil]

Hi

Thanks for your submission, unfortunately for the worshop to work I can't simply merge your pull request, we need to rewrite the history to fix the commit which introduced the issue and I most likely won't be able to do it this week. (same issue for the other PRs)

but thanks again a lot for submitting the PR I really appreciate the effort.

[fpellet]

Integrate in last version (scala/v2.2)