text_rank 1.1.1
See https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjK9tfHxcvMAhVOzGMKHdaQBeEQFggdMAA&url=https%3A%2F%2Fweb.eecs.umich.edu%2F~mihalcea%2Fpapers%2Fmihalcea.emnlp04.pdf&usg=AFQjCNHL5SGlxLy4qmEg1yexaKGZK_Q7IA
See https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjK9tfHxcvMAhVOzGMKHdaQBeEQFggdMAA&url=https%3A%2F%2Fweb.eecs.umich.edu%2F~mihalcea%2Fpapers%2Fmihalcea.emnlp04.pdf&usg=AFQjCNHL5SGlxLy4qmEg1yexaKGZK_Q7IA