assert(isIdentical(log1p(0.0), 0.0));
assert(log1p(1.0).feqrel(0.69314) > 16);
assert(log1p(-1.0) == -real.infinity);
assert(isNaN(log1p(-2.0)));
assert(log1p(real.nan) is real.nan);
assert(log1p(-real.nan) is -real.nan);
assert(log1p(real.infinity) == real.infinity);
The D Programming Language's Unofficial Docs
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Calculates the natural logarithm of 1 + x.
For very small x, log1p(x) will be more accurate than log(1 + x).