code: def div(a,b):
      
      res, fail = divmod(a,b)
      if fail:
          raise ValueError("%r does not divide %r" % (b,a))
      else:
          return res
docs: ``div(a,b)`` is like ``a // b`` if ``b`` devides ``a``, otherwise
      an `ValueError` is raised.
  
      >>> div(10,2)
      5
      >>> div(10,3)
      Traceback (most recent call last):
      ...
      ValueError: 3 does not divide 10