about a subset of NLP which is the supervised learning a part of it so not structured prediction sequence prediction but where you're taking the document as some input and trying to predict some fairly straightforward output about it like is this document spam right and so what this means is that you need to somehow take your document and represent it as a fixed-size vector because I'm not aware of any linear algebra that works on vectors of variable dimensionality and the challenge with this is that documents are of variable length right