Return a list of n-grams (tuples of n successive words) for this
    blob.
    :rtype: List of :class:`WordLists <WordList>`

def ngrams(self, n=3):
    """Return a list of n-grams (tuples of n successive words) for this
    blob.
    :rtype: List of :class:`WordLists <WordList>`
    """
    if n <= 0:
        return []
    grams = [WordList(self.words[i:i+n], parent=self)
                        for i in range(len(self.words) - n + 1)]
    return grams