In biology, it’s pretty common to want to run a single analysis with lots of different choices for the various parameters. Sometimes these choices depend on one another. A common solution for such a situation is to use scripts or makefiles, but we wanted to design something simple and generalizable. Our solution to this is nestly, which makes a nested set of directories, with each level in the hierarchy corresponding to a parameter choice. The input to nestly is a list of lambda functions which make the choices. The result is the nested directory structure, along with JSON files specifying the choices made. Enjoy.