were training you're masking all these input sequences and you know you may sample the masquerades so you you're kind of crunching your inputs but then when you want to train a test time or when you want to transfer to some downstream task it doesn't make sense to corrupt the inputs right because if you were doing some analysis and you mask the token you know this was a mask movie you don't know if it's going to be a great movie or a terrible movie in that mask location so bird just kind of like as a few tricks to minimize this impact