pub fn ln1p(fp: FpUnit, t: f32) -> f32
ln(1 + t) for t ≥ 0: ln u of u = 1 + t rounded, corrected by the rounding’s own error (t − (u − 1))/u, which is exact.
ln(1 + t)
t ≥ 0
ln u
u = 1 + t
(t − (u − 1))/u