at first they have this token and these separate the sentences and then they ask the model to predict a label is next and is next is neck is true if the second sentence follows the first and so it's if it's like a logical continuation and the way you do this unsupervised is really easy you take a big giant corpus and you take a sentence for the first sentence and then 50% of the time you take the next sentence in the corpus and the label is true and 50% of the time you take some random sentence here you say for example the man the man mask to