Thanks to all the past members of the LooPo team who still take the time to help us, and the present members of the team for the wonderful cooperation. In addition, we all want to thank Jean-François Collard for numerous and very helpful explanations and fruitful discussions. Financial support was received from the Deutsche Forschungsgemeinschaft (DFG) by the project RecuR and from the DAAD by the German-French exchange program Procope.