Introduce the built-in range() function.  Don't use np.arange() in example 4--it's really not good practice and I'd rather not promote such usage.  range() here isn't *much* better, though on Python 3 it is since it is the same as xrange().  Really I'd like to use xrange() but we haven't introduced generators yet, and I think that would be throwing too much at people... *sigh*