>> a) Add them as another datatype comletely separate from Elisp >> fixnums, with explicit coercions back and forth. >> >> This, I believe, is possible with minimal changes to the Elisp >> machine, even though it's still a sufficiently hairy job, 'cause >> you need to interface with the garbage collector in a suitable >> way. Hrvoje> Not hairy at all. Have you looked at Hrvoje> DEFINE_LRECORD_IMPLEMENTATION? It allows you to specify a Hrvoje> callback routine for the GC. I wasn't thinking about tracing, but rather about the fact that (according to my totally naive view of how efficient bignum arithmetic works), with some bignum operations, it may not be possible to know the size of the result without actually computing it.

For example, can you shrink an object after creating it?