def overlap(span1, span2):
    """ Return a list `[start,end]` denoting the overlap of two spans.

      Example:

          >>> overlap([1,9], [5,13])
          [5, 9]
  """
    lo = max(span1[0], span2[0])
    hi = min(span1[1], span2[1])
    return [lo, max(lo, hi)]
