I am solving a nonlinear coupled ODE, with 2 functions u and A and a lagrange multiplier r.

I am using Newton Rhapson; I have to control the step size (relaxation parameter) so that the residue doesn’t blow up. This means more no. of steps till convergence, so i am trying to write the output in a xdmf file which i can use as input for the next set of iterations. When I try to store the output using write_checkpoint, I get the following runtime error.

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

*** Error: Unable to find element family.

*** Reason: Element Mixed not yet supported.

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

*** Process: 0

*** DOLFIN version: 2018.1.0

*** Git changeset:

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

My FunctionSpace is created using MixedElements, and I guess the error above remarks that I have to use another route (it’s an implementation issue). It is not clear to me if another route exists. Any method that exists should also be able to read in the written data.

A MWE is presented below:-

```
from dolfin import *
import fenics as fe
import matplotlib.pyplot as plt
#Create mesh and define function space
Lx=10. #length of domain
mesh = fe.IntervalMesh(1000,0,Lx)
x = SpatialCoordinate(mesh)
VA = FiniteElement("CG", mesh.ufl_cell(), 2) #Element for A
Vu = FiniteElement("CG", mesh.ufl_cell(), 2) #Element for u
R = FiniteElement("Real", mesh.ufl_cell(), 0) #Element for Lagrange multiplier
V = FunctionSpace(mesh, MixedElement(VA, Vu, R)) #Creating functionSpace
#Newton Rhapson input
Aur = interpolate( Expression(("1","0.0", "1.5"), degree=2), V)#SC phase as initial cond.
#<NewtonRhapson Code taken out>
##Save solution in a .xdmf file
Aur_out = XDMFFile('test-Const.xdmf')
Aur_out.write_checkpoint(project(Aur,V), "Aur", 0, XDMFFile.Encoding.HDF5, False)
Aur_out.close()
```