next_float, prev_float
Siegfried Rump, Paul Zimmermann, Sylvie Boldo, Guillaume Melquiond.
Computing predecessor and successor in rounding to nearest.
BIT Numerical Mathematics, Springer Verlag, 2009, 49 (2), pp.419-431.
http://dx.doi.org/10.1007/s10543-009-0218-z
https://hal.inria.fr/inria-00337537/document
"The routines deliver the exact answer except for a small range near underflow,
in which case the true result is overestimated by eta [the value added/subtracted]."
Siefried M. Rump, Takeshi Ogita, Yusuke Morikura, Shin'ichi Oishi.
Interval arithmetic with fixed rounding mode.
Nonlinear Theory and Its Applications, IEICE, 2016, 7 (3), pp. 362-373
http://dx.doi.org/10.1587/nolta.7.362
http://www.ti3.tu-harburg.de/paper/rump/RuOgMoOi16.pdf