EDIT: THE ORIGINAL POST DID NOT MAKE MUCH SENSE, SO I ADJUSTED IT
Ok, so I have thought about it, googled it and put some more thought into it. This presentation by the Imperial College in London has a nice example in it, on page 8, Burgers Equations. The first part of their code reads like this:
from dolfin import* n = 30 mesh = UnitInterval(n) V = FunctionSpace(mesh, "CG",2) ic = project(Expression("sin(2*pi*x)"), V) u = Function(ic) u_next = Function(V) v= TestFunction(V) nu = Constant(0.0001) timestep = Constant(1.0/n) F1 = ((u_next - u)/timestep*v)*dx F2 = (u_next*grad(u_next)*v)*dx F3 = (nu*grad(v)*grad(u_next))*dx
On my system this code fails, giving shape mismatch errors for the sums F2 and F3. I don't understand why.