I spent the morning writing about chemical tagging. It appears that The Cannon now delivers precise enough chemical abundance measurements that we can find structure in abundance space (as I discovered in Florida this past weekend). I started to write this up into some kind of paper. In the afternoon, I joined a discussion with the usual mathematical suspects about phase retrieval and set intersection methods. This is very promising! Also, during that discussion, Leslie Greengard handed me a paper by Ayyer et al that poses (pretty much) the question I solved over the break: Can you reconstruct a 3-d image from unknown projections, in each of which you get only one photon?