double atan2(double y, double x);

**atan2** returns the value, in radians, of the inverse circular tangent
of **y** divided by **x** using the signs of **x** and **y** to
compute the quadrant of the return value. The principal value lies in the interval
[−π, +π] radians.
If **x** = **y** = 0, **errno** is set to **EDOM** and **atan2** returns **HUGE_VAL**.

**atan2**(**x**, NaN) is NaN.

**atan2**(NaN, **x**) is NaN.

**atan2**(±0, +(anything but NaN)) is ±0.

**atan2**(±0, −(anything but NaN)) is ±π.

**atan2**(±(anything but 0 and NaN), 0) is ±½π.

**atan2**(±(anything but ∞ and NaN), +∞) is ±0.

**atan2**(±(anything but ∞ and NaN), −∞) is ±π.

**atan2**(±∞, +∞) is ±¼π.

**atan2**(±∞, −∞) is ±¾π.

**atan2**(±∞, (anything but 0, NaN, and ∞)) is ±½π.