here they say why don't we consider all possible orderings so basically what we will do is if this sample comes up New York is a city all right what I can do is I can define an ordering let's say I always want to predict two words so the bird typically masks out about 15% of its input to be predicted and here let's say we'll mask out 20% which two words so of this sequence will mask two words and ask the model to predict it that that will be our our pre training objective the first time this sample comes up from the data set