Note that a builtin 'slice' does not contain any data (it is just a container for 3 arguments), but here an 'ObjSlice' contains data (a reference to an object). So it is a bit different. I was not sure about the name, though, either.

I placed it in helpers.py to avoid a cyclic reference. Neurons in nonlinearities.py needs ObjSlice. And objects.py depends on nonlinearities.py. The alternatives would be to: 
(1) Make a new file for ObjSlice
(2) Put ObjSlice in nonlinearities.py
(3) Remove the cyclic reference somehow...?