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Structure-based gradient or Jacobian functions, including Jacobians of general constraints, must return the partial derivatives as fields of a structure under the respective parameter names. For gradients, the partial derivatives are scalar. For Jacobians, the partial derivatives must be column vectors.
Structure-based Hessian functions must return the 2nd derivatives as
subfields in a two-level structure of parameter names. For example, if
the parameter names are a and b, the 2nd derivative with
respect to a and b must be in the field
returned_structure.a.b or returned_structure.b.a (there is
no need to specify both).