Why is 'fem.assemble_scalar(fem.form(ufl.cross(ufl_vec, sigma(u)*n)[1]*ds(ds_section)))' throwing "ValueError: Index out of bounds."

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What is unclear to me what you can extract from the [1] component of a cross-product of a 2D vector. It would at most be the third component of the cross product that would be interesting, ref: https://www.quora.com/Can-you-cross-product-2D-vectors.

What I would assume, is that you can implement the 2D cross product yourself:

   def cross_2D(x, y):
        return x[0] * y[1] - x[1] * y[0]
    sgn = sigma(u) * n
    ufl_vec = ufl.as_vector([x_pos, 3.9])
    My = fem.assemble_scalar(fem.form(cross_2D(ufl_vec, sigma(u) * n) * ds(ds_section)))

However, your code is rather long, and doesn’t get straight to the point, which is:
How to compute a cross product of 2D vectors with ufl.

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