Atan
Function 
Atan(number)

Atan is included by default in each file as part of the Math library.
See also:
Use
The Atan() function returns the arctangent of a given unitless number. The result is given in radians, but it can be converted to degrees using "to deg".
Atan() will always return an angle in the 1st and 4th cartesian quadrants. So, when using negative numbers, it can be important to use Atan2.
Atan() also works with complex numbers.
Examples of trig arc functions
Acos(pi/4) = .667 rad
Acos(0.5) to deg = 60 deg
Atan(3ft/5ft) = 0.540 rad
Asin(1 + 2i) = (0.427 + 1.529 i) rad