question about project_a and project_b

[llianxu]

Thanks for your impressive work!
I have some question about out_jac , Why this argument makes project_a and project_b evaluate differently

[Lakonik]

Calculating the Jacobian requires the projection function to output x3d_rot. If Jacobian is not needed, x3d_rot would be unnecessary, so project_b implements another projection function without calculating x3d_rot, which is slightly faster.