Let's say I have a Mixed FunctionSpace
mixed_element = MixedElement(element, element, element)
Q = FunctionSpace(mesh,element=mixed_element)
u = Function(Q)
du = TrialFunction(Q)
G = F(u) #some Coefficient which depends on u
dG = derivative(G,u,du, coefficient_derivatives=some_dict)
How do I define my custom derivative coefficient to G? I am assuming this would be an Expression of dimension 3 in this case. Or is it instead a tuple (dG/du[0], dG/du[1], dG/du[2])?
