The Wrath of Janus

What went wrong?

:- janus
(list 31
      2e+34
      -1.2345678901235e+80
      2749
      -2939234
      -2e+33
 -->  +3.2e+270
      17
 -->  -2.4e+270
      4.2344294738446e+170
      1
 -->  -0.8e+270
      0
      99)

Look at the three indicated expressions in the list. These are the largest entries, ludicrously dwarfing everything else by a factor of 10^100. 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 small stuff.

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