2.3 word embeddings
    (1) analogies using word vectors
      Example: man to woman is king to ? queen!
      However, these analogies are more likely to exist in the original space (300-dimension).
      After **t-SNE**, which is highly nonlinear mapping to **2D** space, these analogies usually don't hold.

2.5 word2vec
    (1) skip-gram model