'Highlight the given item.
:raise ValueError: If ``item`` is not contained in the list.'

def mark(self, item):

    self._marks[item] = 1
    index = self._items.index(item)
    (start, end) = (('%d.0' % (index + 1)), ('%d.0' % (index + 2)))
    self._textwidget.tag_add('highlight', start, end)
