mortar_mesh_mwe

from dolfin import *
from xii import *
N = 20
EPS = 1E-3

_mesh = RectangleMesh(Point(0, -1), Point(1, 1), N, 2 * N)

stokes_subdomain = dolfin.CompiledSubDomain(
    "x[1] > 0", eps=EPS
)

subdomains = MeshFunction('size_t', _mesh, _mesh.topology().dim(), 0)

# Awkward marking
for cell in cells(_mesh):
    x = cell.midpoint().array()
    if stokes_subdomain.inside(x, False):
        subdomains[cell] = 1
    else:
        subdomains[cell] = 0
        
submeshes, interface, _ = mortar_meshes(
    subdomains, range(2), strict=True, tol=EPS)