I also set create-lockfiles to nil. I think you can theoretically keep the lockfiles and the clean directory by using lock-file-name-transforms to place the lockfiles somewhere sensible but I didn't bother.

Without this I had to be careful not to acciddentally commit stuff like ".#filename.txt".

SBCL uses a single zero bit to tag integers. This trick means the representation of n is just 2n, so you can add the values directly without any decoding.

It obviously also means that all the other tag values have to use 1 as the last bit.