'Inserts a new rule that updates words with tag1 to tag2,
given constraints x and y, e.g., Context.append("TO < NN", "VB")'

def insert(self, i, tag1, tag2, cmd=u'prevtag', x=None, y=None):

    if ((u'    <    ' in tag1) and (not x) and (not y)):
        (tag1, x) = tag1.split(u'    <    ')
        cmd = u'prevtag'
    if ((u'    >    ' in tag1) and (not x) and (not y)):
        (x, tag1) = tag1.split(u'    >    ')
        cmd = u'nexttag'
    lazylist.insert(self, i, [tag1, tag2, cmd, (x or u''), (y or u'')])
