* add exception handling to Graph_batch.parse (so that the error message can be more precise)
 -> so that open and modification can be partially accepted, but still be corrected

* rewrite nearest_functions_ so that it takes only functions that are inside the rectangle
* zoom around the clicked point when the area selected is too small ??
* pour la racine d'une fonction, pouvoir preciser la precision
* (code) unify the start -f option and the open_file functions
* history of the typed functions (combo box?)
* gridded zoom ? (attach the corners to round values...)
* zoom, maybe any key while choosing zooming rectangle should cancel ??
* be able to modify the objects (plots, points, tangents...) by clicking on them

* print preview (choose the size and the location)
* save the given function (eg: x+y=2) instead of the isolated one (eg: y=-x+2)
* eval being able to be evaluate expr with many vars being given
* expand needs work
* colct needs work