'One left & one right token, both case-normalized.  Skip over
non-sentence-final punctuation.  Used by the ``ContextIndex``
that is created for ``similar()`` and ``common_contexts()``.'

def _context(self, tokens, i):

    j = (i - 1)
    while ((j >= 0) and (not self._CONTEXT_RE.match(tokens[j]))):
        j -= 1
    left = (tokens[j] if (j != 0) else u'*START*')
    j = (i + 1)
    while ((j < len(tokens)) and (not self._CONTEXT_RE.match(tokens[j]))):
        j += 1
    right = (tokens[j] if (j != len(tokens)) else u'*END*')
    return (left, right)
