How would I write vectors into a neanderthal matrix?

I am reading them from a text file... my matrix should be a dense matrix sized ~500,000 over 300 (a word embedding, which I'm loading from a file)

also, I'm not seeing where to use or require float-factory from. oh, float-factory is just a documentation error, it is now native-float isn't it.

could you kindly suggest something? I tend to think I should initialize it in one shot, hoping that mapcating the file's lines into the neanderthal matrix initialization would not materialize the entire file in memory. I have found for getting a vector of a matrix, but not for setting it. Anything more efficient than using alter!? Is multiplication with an extremely sparse matrix the way to write specific vectors into/from the matrix?? or is there anything more direct? how bad do you think would it be building and using a sparse "selection matrix" for updating/extracting specific rows?