'Remove highlighting from the given item; or from every item,
if no item is given.
:raise ValueError: If ``item`` is not contained in the list.
:raise KeyError: If ``item`` is not marked.'

def unmark(self, item=None):

    if (item is None):
        self._marks.clear()
        self._textwidget.tag_remove('highlight', '1.0', 'end+1char')
    else:
        index = self._items.index(item)
        del self._marks[item]
        (start, end) = (('%d.0' % (index + 1)), ('%d.0' % (index + 2)))
        self._textwidget.tag_remove('highlight', start, end)
