arc_tangent
This function returns the principal value of the arc tangent of x, expressed in radians.
double arc_tangent(double x)
Parameters:
x
The number to calculate.
Return value:
Principal arc tangent of x, in the interval [-pi/2,+pi/2] radians.
Remarks:
In trigonometrics, arc tangent is the inverse operation of tangent.
Notice that because of the sign ambiguity, a function cannot determine with certainty in which quadrant the angle falls only by its tangent value.
Example:

    void main()
    {
    alert("arc_tangent test", "Arc tangent of -1 is "+arc_tangent(-1)+".");
    alert("arc_tangent test", "Arc tangent of 0 is "+arc_tangent(0)+".");
    alert("arc_tangent test", "Arc tangent of 1 is "+arc_tangent(1)+".");
    }