I would like to use a lumped mass matrix as part of a time dependant problem. There is a solution (https://fenicsproject.org/qa/4284/mass-matrix-lumping/) that gives some ideas but I don’t see how to apply this to define my variational form.

Consider this (I know very silly) example of a L2 projection using mass lumping.

```
from fenics import *
mesh = UnitIntervalMesh(5)
P1 = FiniteElement("Lagrange", mesh.ufl_cell(), 1)
V = FunctionSpace(mesh, P1)
u = TestFunction(V)
v = TrialFunction(V)
u_old = interpolate(Constant(2.0), V)
mass_form = v*u*dx()
mass_action_form = action(mass_form, Constant(1))
M_lumped = assemble(mass_form)
M_lumped.zero()
M_lumped.set_diagonal(assemble(mass_action_form))
# TODO what to put here?
F = M_lumped * (u - u_old)
u = Function(V)
solve(F==0, u)
print(u.vector().get_local())
```

Is there a way to use M_lumped here or do I need to do something else?

Thanks in advance for any help