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[oelgaard <k.b.oelgaard@xxxxxxxxxx>]

** Changed in: ffc
       Status: Confirmed => Fix Released

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Problems parsing form file with diff and derivative
https://bugs.launchpad.net/bugs/426512
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Status in FEniCS Form Compiler: Fix Released

Bug description:
FFC has problems parsing the following form file. As I have pointed out in the code, I need to use the following:

from ufl.algorithms import expand_derivatives, strip_variables
L = strip_variables(expand_derivatives(L_temp))
a = strip_variables(expand_derivatives(a_temp))

to get it to compile via FFC. This is not needed for SFC, and while SFC takes a few seconds (without these lines), FFC takes 6m 25s)

Also, FFC doesn't like fractional exponents which is pointed out in bug #423398.

# Finite strain elasticity

# Function space
vector = VectorElement("Lagrange", "tetrahedron", 1)

# Functions
v  = TestFunction(vector)   # Test function
du = TrialFunction(vector)  # Incremental displacement
B  = Function(vector)       # Body force
u  = Function(vector)       # Displacement at previous iteration

# Kinematics
I = Identity(3)
F = I + grad(u).T
C = F.T*F
J = sqrt(det(C))
Fbar = J**(-1./3.)*F  # Does not like fractional exponents
Cbar = J**(-2./3.)*C  # Does not like fractional exponents

E = (J*C - I)/2       # Want to try Cbar here, but using J*C instead
E = variable(E)

# Material law
mu    = Constant("tetrahedron")
lmbda = Constant("tetrahedron")

psi = lmbda/2*(tr(E)**2) + mu*tr(E*E)
S = diff(psi, E)
P = F*S

# Balance of momentum in the reference configuration
L_temp = inner(P, grad(v).T)*dx - inner(B, v)*dx
a_temp = derivative(L_temp, u, du)

# FIXME: Shouldn't need to do the following
from ufl.algorithms import expand_derivatives, strip_variables
L = strip_variables(expand_derivatives(L_temp))
a = strip_variables(expand_derivatives(a_temp))