'Appends the given Document to the model.
If Model.weight != TF, the cache of vectors and cosine similarity is cleared
(feature weights will be different now that there is a new document).'

def append(self, document):

    if (not isinstance(document, Document)):
        document = Document(document)
    if (document.name is not None):
        self._index[document.name] = document
    document._model = self
    list.append(self.documents, document)
    if (self._weight not in (TF, BINARY, None)):
        self._update()
