Hello,

I am setting up a problem with `MixElement()`

of 2 functions, the Dirichlet BC only applies to only one of the functions (one that corresponds to `P1`

), how should I implement that? See the following code:

```
from dolfin import *
mesh = IntervalMesh(20, 0, 1)
P1 = FiniteElement("CG", mesh.ufl_cell(), 1)
P2 = FiniteElement("CG", mesh.ufl_cell(), 1)
element = MixedElement([P1,P2])
V = FunctionSpace(mesh, element)
def Dirichlet_Left(x, on_boundary):
return on_boundary and x[0] < DOLFIN_EPS
bc = DirichletBC(V, Constant(100.), Dirichlet_Left)
```

The last line gives this error:

RuntimeError:

*** -------------------------------------------------------------------------

*** DOLFIN encountered an error. If you are not able to resolve this issue

*** using the information listed below, you can ask for help at

*** fenics-support@googlegroups.com

*** Remember to include the error message listed below and, if possible,

*** include a

minimalrunning example to reproduce the error.

*** -------------------------------------------------------------------------

*** Error: Unable to create Dirichlet boundary condition.

*** Reason: Expecting a vector-valued boundary value but given function is scalar.

*** Where: This error was encountered inside DirichletBC.cpp.

*** Process: 0

*** DOLFIN version: 2019.1.0

*** Git changeset: 15b823f2c45d3036b6af931284d0f8e3c77b6845

*** -------------------------------------------------------------------------