;; take up about the same space as corresponding fixnums, there
;; should be no way that we fall through to this case: any shift
;; right by a bignum should give zero. But let's check anyway:
;; take up about the same space as corresponding fixnums, there
;; should be no way that we fall through to this case: any shift
;; right by a bignum should give zero. But let's check anyway: