[firedrake] Surface integrals on custom mesh

Dear all, I have the mesh defined in the file attached. I can successfully import it and use it with Firedrake, but how can I define a surface integral? For example, I want to somehow define a "ds" which will denote the left boundary of my domain, corresponding to y=0. The nodes that will define this "ds" are conveniently the first 20 nodes in the mesh file attached, with y=0 (the second column). Thanks, Anna.