http://web.stanford.edu/class/cs224n/lectures/cs224n-2017-lecture17.pdf
http://web.stanford.edu/class/cs224n/lectures/cs224n-2017-lecture18.pdf
http://web.stanford.edu/class/cs224n/lectures/cs224n-2017-lecture17.pdf
http://web.stanford.edu/class/cs224n/lectures/cs224n-2017-lecture18.pdf