What went wrong?
(define janus (list 99 --> 3.2e+270 -99 --> -2.4e+270 1234 4 --> -8e+269 -4))Look at the three indicated expressions in the list. These are the largest entries, ludicrously dwarfing everything else by a factor of around 10265. Notice that they cancel each other out.
Here's a smaller list that has the exact same problem:
(list 1 +1e20 -1e20 99)Add up the list in each direction yourself, remembering to only worry about the most significant 15 digits, ignoring the relatively small.