docs: def single(a, b, distance_function):
      """
      Given two collections ``a`` and ``b``, this will return the distance of the
      points which are closest together.  ``distance_function`` is used to
      determine the distance between two elements.
  
      Example::
  
          >>> single([1, 2], [3, 4], lambda x, y: abs(x-y))
          1  # (distance between 2 and 3)
      """
      left_a, right_a = min(a), max(a)
      left_b, right_b = min(b), max(b)
      result = min(distance_function(left_a, right_b),
                   distance_function(left_b, right_a))
      return result