Hi, for 4×4 system of PDEs, I considered trial and test functions mixed of 4 function spaces as the following:

V1 = FunctionSpace(mesh, "CG", deg)
V2 = VectorFunctionSpace(mesh, "CG", deg)
V3 = TensorFunctionSpace(mesh, "CG", deg)
P = FunctionSpace(mesh, "CG", deg)
V = V1 * V2 * V3 * P

u, g, H, p = TrialFunction(V)

phi1, phi2, phi3, q = TestFunction(V)

but I got the following error

Traceback (most recent call last):
  File "test1.py", line 58, in <module>
    u, g, H, p = TrialFunction(V)
ValueError: too many values to unpack (expected 4)

Many thanks if someone could give me some  direction about that.

Best,

Amireh