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> Oops, I get it now.
Okay. :-)
Just for the record, for anyone encountering this issue in the future, here's the relevant extract from section 6.3 of IEEE 754-2008:
"""
When the sum of two operands with opposite signs (or the difference of two operands with like signs) is exactly zero, the sign of that sum (or difference) shall be +0 in all rounding-direction attributes except roundTowardNegative; under that attribute, the sign of an exact zero sum (or difference) shall be −0. However, x + x = x − (−x) retains the same sign as x even when x is zero.
"""
It doesn't cover the "sum with like signs" or "difference with opposite signs" cases, I suppose because it should be obvious what happens in those cases.