Fixed KeyFrame in Local BA?

[YipuZhao]

I was trying to do double-window optimization with ICE-BA, since it is the most efficient & incremental solver so far. However, according to my quick glance of the code, it doesn't seem to support fixed keyframe in local BA (and map points thereafter, because of the inverse-depth parameterization).

I was wondering if there's any (relatively) easy modification to fix a subset of keyframes in local BA, while the rest of them are flexible. Appreciate any suggestion!