Error for more or less standard variational formulation using two cross products

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From the ufl documentation: Form language — Unified Form Language (UFL) 2019.2.0.dev0 documentation

The operator cross accepts as arguments two logically vector-valued expressions and returns a vector which is the cross product (vector product) of the two vectors:

𝚌𝚛𝚘𝚜𝚜(𝚟,𝚠)↔v×w=(v1w2−v2w1,v2w0−v0w2,v0w1−v1w0)

Note that this operator is only defined for vectors of length three.