@stloyd Of course we can not use spaces... My goal was to able to define several data sets in the same class. One data set, one function that was simple and "quite" clean. Other possible approaches are : 
* save all data sets in one array, the simplest. But this array will become unreadable with a lot of dataset (or with big data set). 
* only one data set per class that implements a `DataSetInterface`. When we want to load a data set, we look for all classes that implements `DataSetInterface` and we load the right one. Surely cleaner. Overkill for only this ?

Opinions ?