Restrict an expression twice

Question

I ve tried to add this integral but comes with error

Exception: Expression is restricted twice

+dot(u0*n,avg(D*grad(v('-'))))*ds(3)

Comments

Can you write the mathematical integral you're trying to formulate in ufl?

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$\int an \cdot \lbrace D \nabla u \rbrace d S(3)$

It can be seen in page 9 from article. It is a particle of equation 3.9 in $ \Gamma_I$ interface

Answer 1

I ve open the expression to solve it. If any have the same problem. Look below:

-(0.5)dot(D("-")grad(v)("-"),u0n("-"))dS(3)-(0.5)dot(D("+")grad(v)("+"),u0n("+"))dS(3)
-(0.5)dot(D("+")grad(v)("+"),u0n("-"))dS(3)-(0.5)dot(D("-")grad(v)("-"),u0n("+"))dS(3)

But the problem has been solved for

-(0.5)dot(D("-")grad(v)("-"),u0n("-"))dS(3)