Bit vectors are also one of the rare types that are
individually. Consequently, while sweeping, all further needless
bit vectors must be freed by hand. This is done, as one might imagine,
the expected way: since they are all registered in a list called
all_bit_vectors, all elements of that list are traversed,
all unmarked bit vectors are unlinked by calling
xfree and all of
them become unmarked.
In addition, the bookkeeping information used for garbage
collector’s output purposes is updated.