Function: acos
Section: transcendental
C-Name: gacos
Prototype: Gp
Help: acos(x): inverse cosine of x.
Doc: principal branch of $\text{cos}^{-1}(x)$, i.e.~such that
 $\text{Re(acos}(x))\in [0,\pi]$. If $x\in \R$ and $|x|>1$, then
 $\text{acos}(x)$ is complex.
