title: Gensim Word2vec Freeze some wordvectors and Update others
body: <p>Regarding word2vec with gensim,
  Suppose you already trained a model on a big corpus, and you want to update it with new words from new sentences, but not update the words which already have a vector.
  Is it possible to freeze the vectors of some words and update only some chosen words (like the new words) when calling <code>model.train</code> ?
  Or maybe is there a trick to do it ?
  Thanks.</p>