Dear Fenics users,

I’d like to apply an expression on a function defined on a Function space V, in order to have polar coordinates as unknown in Fenics but classic coordinates in the variational formulation of my problem.

```
class MyRotation(UserExpression):
def eval(self,values,x):
r=10
x0=0
y0=-10
values[0]=x0+(x[1]-y0)*sin(x[0]/r) -x[1]
values[1]=y0+(x[1]-y0)*cos(x[0]/r) -x[2]
def value_shape(self):
return (2,)
Ve = VectorElement("Lagrange", mesh.ufl_cell(), 1)
v=Function(V)
f=MyRotation(v) #doesn't work!!!
```