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["Robert Kirby" <robert.c.kirby@xxxxxxxxx>]

Hi all, I'm trying to do a survey of all the different ways of enforcing
incompressibility in the Navier-Stokes equations, and FFC/FIAT of course has
the most advanced element capabilities for doing this...

Unfortunately, I have hit a snag (either in my ignorance or through a bug)
in my grand scheme, starting with the Oseen equations (fix the convective
velocity for fixed points, don't do the full Newton linearization).

It seems that if you put a vector element into a mixed element and then take
it back out, you can no longer index it.

Putting the following in a .form (everything that doesn't involve pressure
spaces) works beatifully
#start .form file here
vel_element = VectorElement("Lagrange","triangle",2)
u = TrialFunction( vel_element )
v = TestFunction( vel_element )
u0 = Function( vel_element )

a = D(v[i],j)*D(u[i],j)*dx + v[i]*u0[j]*D(u[i],j)*dx
#end .form file

On the other hand, if I do:
# start new .form file here
vel_element = VectorElement("Lagrange","triangle",2)
pre_element = FiniteElement("Lagrange","triangle",1)
element = vel_element + pre_element
(u,p) = TrialFunctions( element )
(v,q) = TestFunctions( element )
u0 = Function( vel_element )

a = D(v[i],j)*D(u[i],j)*dx + v[i]*u0[j]*D(u[i],j)*dx
# end new .form file

I get an error:

Preprocessing form file: Oseen.form --> Oseen.py

*** object cannot be interpreted as an index
*** To get more information about this error, rerun ffc with the option -d1.

I've attached the output using -d1.

Any corrections either of my stupidity or of FFC would be appreciated :)

Rob