Consing sounds like it's allocating bignums. My guess is that you're using a 32-bit build of SBCL. In that case, fixnums only go up to something like 2^30, an

Consing sounds like it's allocating bignums. My guess is that you're using a 32-bit build of SBCL. In that case, fixnums only go up to something like 2^30, and arithmetic with larger numbers will allocate memory. Can you check? * (log most-positive-fixnum 2) ;on 64-bit 62.0