'Remove all tags (except ``arrow`` and ``sel``) from the given
line of the text widget used for editing the productions.'

def _clear_tags(self, linenum):

    start = ('%d.0' % linenum)
    end = ('%d.end' % linenum)
    for tag in self._textwidget.tag_names():
        if (tag not in ('arrow', 'sel')):
            self._textwidget.tag_remove(tag, start, end)
