trajectory and matrix of BGGEXP element

[Bingfeng]

Dear Michael,

I have been learning the BGGEXP and LGBEND methods recently,
and I have several questions that I don't understand.

1.
According to my understanding, the tracking process in BGGEXP is:
1) project particles from the ideal entrance edge to the start of the GGE field (as a drift);
2) track particles through the GGE field;
3) project particles from the end of the GGE field to the ideal exit edge.
However, the start of the trajectory output from the BGGEXP element is not consistent with the ideal trajectory.
The deviation at the ideal exit edge is also not consistent with Δx in the zero-order transfer map.

Attached is a comparison of trajectories for a rectangular bending magnet.
mpr data: C: 1.180117e-05 1.312073e-04 0.000000e+00 0.000000e+00 1.800011e-01 0.000000e+00

2.
For the LGBEND element, there are two parameters: OPTIMIZE_FSE and COMPENSATE_KN.
I don’t understand why the strength is optimized by default,
rather than using the raw field distribution to evaluate the influence of the original field errors.

I would greatly appreciate it if you could help me with these questions.
Thank you very much for your time and support.