I am solving a diffusion equation in the domain and I would like to set initial conditions such that the domain is different from the blue boundary. I have the relevant boundary labelled and I can find them using:

comm = MPI.COMM_WORLD
with io.XDMFFile(comm, "mesh/laminate/tria.xdmf", "r") as xdmf:
domain = xdmf.read_mesh(cpp.mesh.GhostMode.shared_facet, name="Grid")
ct = xdmf.read_meshtags(domain, name="Grid")
domain.topology.create_connectivity(domain.topology.dim, domain.topology.dim - 1)
with io.XDMFFile(comm, "mesh/laminate/line.xdmf", "r") as xdmf:
ft = xdmf.read_meshtags(domain, name="Grid")
left_facet = ft.find(markers.left_bndry)
# set for bulk
u.x.array[:] = 10
#?? change value to 0 on left boundary

However, I am unclear on how to change the value to 0 on the left boundary.