Dear All,

I have the following mesh from gmsh (see attached).

I can read it just fine:
mesh = Mesh("lshape.msh")

but when I try to solve equation on this mesh it fails to apply boundary
conditions (because boundaries isn't marked in the msh file, I guess):

---------------------------------------------------------------------------
LookupError                               Traceback (most recent call last)
<ipython-input-17-ae4ad3326f99> in <module>()
      6 fun0 = None
      7 cff = 0.002
----> 8 sol = get_sol(frm, rhs)
      9 fun0 = get_functional(sol, f)

<ipython-input-15-e7cda2e05bae> in get_sol(frm, rhs)
      4     sol = Function(V)
      5     #solve(frm, sol, rhs, solver_parameters={'ksp_type': 'cg'})
----> 6     solve(frm, sol, rhs, solver_parameters={"ksp_type":
"preonly", "pc_type": "lu"})
      7
      8     return sol

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/solving.pyc
in solve(*args, **kwargs)
    121     else:
    122         # Solve pre-assembled system
--> 123         return _la_solve(*args, **kwargs)
    124
    125

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/solving.pyc
in _la_solve(A, x, b, **kwargs)
    209     solver = ls.LinearSolver(A, solver_parameters=solver_parameters,
    210                              nullspace=nullspace,
--> 211                              options_prefix=options_prefix)
    212
    213     solver.solve(x, b)

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/linear_solver.pyc
in __init__(self, A, P, solver_parameters, nullspace, options_prefix)
     87         # applied
     88         # Force evaluation here
---> 89         self.ksp.setOperators(A=self.A.M.handle, P=self.P.M.handle)
     90
     91     @cached_property

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/matrix.pyc
in M(self)
    143             just need a handle on the :class:`pyop2.Mat` object it's
    144             wrapping, use :attr:`_M` instead."""
--> 145         self.assemble()
    146         # User wants to see it, so force the evaluation.
    147         self._M._force_evaluation()

<decorator-gen-414> in assemble(self)

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/utils.pyc
in wrapper(f, *args, **kwargs)
     60         opts["type_check"] = safe
     61         try:
---> 62             return f(*args, **kwargs)
     63         finally:
     64             opts["type_check"] = check

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/matrix.pyc
in assemble(self)
     72             return
     73         self._bcs_at_point_of_assembly = copy.copy(self.bcs)
---> 74         self._assembly_callback(self.bcs)
     75         self._assembled = True
     76

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/assembly_cache.pyc
in inner(bcs)
    356             return r
    357
--> 358         r = thunk(bcs)
    359         if isinstance(r, float):
    360             # 0-form case

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/assemble.pyc
in thunk(bcs)
    328                 tensor_arg = mat(lambda s: get_map(s, tsbc,
decoration),
    329                                  lambda s: get_map(s, trbc,
decoration),
--> 330                                  i, j)
    331             elif is_vec:
    332                 tensor_arg = vec(lambda s: get_map(s), i)

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/assemble.pyc
in mat(testmap, trialmap, i, j)
    189         def mat(testmap, trialmap, i, j):
    190             return tensor[i, j](op2.INC,
--> 191
(testmap(test.function_space()[i])[op2.i[0]],
    192
trialmap(trial.function_space()[j])[op2.i[1]]),
    193                                 flatten=True)

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/assemble.pyc
in <lambda>(s)
    326             # Output argument
    327             if is_mat:
--> 328                 tensor_arg = mat(lambda s: get_map(s, tsbc,
decoration),
    329                                  lambda s: get_map(s, trbc,
decoration),
    330                                  i, j)

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/assemble.pyc
in get_map(x, bcs, decoration)
    280
    281                 def get_map(x, bcs=None, decoration=None):
--> 282                     return x.cell_node_map(bcs)
    283
    284             elif integral_type in ("exterior_facet",
"exterior_facet_vert"):

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/functionspaceimpl.pyc
in cell_node_map(self, bcs)
    411                              "cell_node",
    412                              self.offset,
--> 413                              parent)
    414
    415     def interior_facet_node_map(self, bcs=None):

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/functionspacedata.pyc
in get_map(self, V, entity_set, map_arity, bcs, name, offset, parent)
    485             # Cache miss.
    486             # Any top and bottom bcs (for the extruded case) are
handled elsewhere.
--> 487             nodes = [bc.nodes for bc in lbcs if bc.sub_domain
not in ['top', 'bottom']]
    488             decorate = False
    489             if nodes:

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/pyop2/utils.pyc
in __get__(self, obj, cls)
     62         if obj is None:
     63             return self
---> 64         obj.__dict__[self.__name__] = result = self.fget(obj)
     65         return result
     66

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/bcs.pyc
in nodes(self)
    152                 fs.exterior_facet_boundary_node_map(
    153                     self.method).values_with_halo.take(
--> 154
fs._mesh.exterior_facets.subset(self.sub_domain).indices,
    155                     axis=0))
    156

/home/george/polygon/firedrake/local/lib/python2.7/site-packages/firedrake/mesh.pyc
in subset(self, markers)
    125                     raise LookupError(
    126                         '{0} is not a valid marker'.
--> 127                         format(marker))
    128
    129             # build a list of indices corresponding to the
subsets selected by

LookupError: 4 is not a valid marker

So, I need to somehow mark boundaries, but I cannot find it out, how.
I've tried
https://openfoamwiki.net/index.php/2D_Mesh_Tutorial_using_GMSH but step
4 does not work for me -- I simply cannot select line with gui, and it
is not clear how to select boundaries editing mesh file.

Sincerely,
George