```atan()``` returns the angle whose trigonometric arctangent is ```y,x``` or ```y_over_x```, depending on which overload is invoked. In the first overload, the signs of ```y``` and ```x``` are used to determine the quadrant that the angle lies in. The values returned by atan in this case are in the range -PI and PI. Results are undefined if ```x``` is zero.
For the second overload, ```atan()``` returns the angle whose tangent is ```y_over_x```. Values returned in this case are in the range -PI to PI.