Function: atan
Section: transcendental
C-Name: gatan
Prototype: Gp
Help: atan(x): inverse tangent of x.
Doc: principal branch of $\text{tan}^{-1}(x)$, i.e.~such
 that $\text{Re(atan}(x))\in{} ]-\pi/2,\pi/2[$.
