Hi Lawrence,

I had already tried this based on examples from firedrake/utility_meshes.py, but I get the following error:

Traceback (most recent call last):
  File "mesh.py", line 39, in <module>
    V = FunctionSpace(mesh, "CG", 1)
  File "/Users/matak/firedrake/lib/python2.7/site-packages/firedrake/functionspace.py", line 636, in __new__
    mesh.init()
  File "/Users/matak/firedrake/lib/python2.7/site-packages/firedrake/mesh.py", line 750, in init
    self._callback(self)
  File "/Users/matak/firedrake/lib/python2.7/site-packages/firedrake/mesh.py", line 1040, in callback
    dim=geometric_dim)
  File "/Users/matak/firedrake/lib/python2.7/site-packages/firedrake/functionspace.py", line 695, in __new__
    self = super(VectorFunctionSpace, cls).__new__(cls, mesh_t, element, name=name, shape=(dim,))
  File "/Users/matak/firedrake/lib/python2.7/site-packages/firedrake/functionspace.py", line 140, in __new__
    dmplex.get_facet_nodes(mesh.exterior_facets.facet_cell,
  File "/Users/matak/firedrake/lib/python2.7/site-packages/pyop2/utils.py", line 64, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/Users/matak/firedrake/lib/python2.7/site-packages/firedrake/mesh.py", line 471, in exterior_facets
    boundary_ids, unique_markers=unique_ids)
  File "/Users/matak/firedrake/lib/python2.7/site-packages/firedrake/mesh.py", line 61, in __init__
    "Every marker has to be contained in unique_markers"
AssertionError: Every marker has to be contained in unique_markers

A sample code can be found attached.

Best, Anna.