I have a feeling that *should* not be used anymore since we now signal top and bottom iteration in the correct way using keyword arguments. 
So in host where we say:

a_bcs = .... . _extruded_bcs

it should actually become

a_bcs = (op2.ON_BOTTOM==self._iterate, op2.ON_TOP==self._iterate)

This is because the function that generates the code assumes there is a tupe of bool values of the type: (bottom, top). The above replacement works because now we have separate par loops for top and bottom.