float atanhf(float x);
atanhf returns the inverse hyperbolic tangent of val.
If |x| ≥ 1, errno is set to EDOM and atanhf returns HUGE_VALF.
If |val| > 1 atanhf returns NaN. If val is NaN, atanhf returns that NaN. If val is 1, atanhf returns ∞. If val is ?1, atanhf returns ?∞.