[firedrake] assign different values on subdomains

Hi all, I would like to assign different values of some function u on subdomains of a 2-D domain. More precisely, I want to have u equal to 1 for y > x and equal to 0 otherwise. For that reason I guess one needs to be able to define subdomains. Besides the Issue on https://github.com/firedrakeproject/firedrake/issues/723 I didn't find any helpful information online. Can someone help out please? I attached the fenics version of what I want, hoping that might help! All the best, Tobias