For a toy problem on a way to a more complicated system of PDEs, I tried to solve a variational problem of the form

a(u,v) = \int (u \times v, u \times v) dx.

Edit: the above formulation is not well defined in the sense of FEM. What I rather meant was

a(u,v) = \int (u \times g, u \times v) dx.

for some function g.

However I do end up with the Error, when I try to solve the variational problem with a non-linear solver,

UFLException: Index out of bounds.

which seems to happen in the UFL package.

My code is

```
from fenics import *
mesh = UnitSquareMesh(2, 2)
deg = 1
V=VectorFunctionSpace(mesh, "P",deg, dim=2)
u = Function(V)
v = TestFunction(V)
L = inner(cross(u, v) , cross(u, v) )*dx
solve(L==0, u)
```

I have to say I am quite overwhelmed with the error message because it does not really hint where I might have made a mistake. To me the variational problem seems well defined with a trivial solution, therefore the issue must occur during the assembly.

Would be very happy about every hint for my problem