Function: atanh
Section: transcendental
C-Name: gath
Prototype: Gp
Help: atanh(x): inverse hyperbolic tangent of x.
Doc: principal branch of $\text{tanh}^{-1}(x)$, i.e.~such
 that $\text{Im(atanh}(x))\in{} ]-\pi/2,\pi/2]$. If $x\in \R$ and $|x|>1$ then
 $\text{atanh}(x)$ is complex.
