Radian system of measuring angles

All of the built-in trigonometric functions in FreeBASIC express angles in 
radians.

Let pi the constant equal to the ratio of the circumference of a circle to 
its diameter. It can be calculated programmatically by multiplying the 
arctangent of 1 by 4.
A full circle is divided into 2 * pi radians or 360 degrees, which leads to 
the following conversions:

   pi = Atn(1) * 4
   radians = degrees * pi / 180
   degrees = radians * 180 / pi

These 3 formulas can be expressed by using macros:

   #define   pi          ( Atn(1) * 4 )
   #define   radian(x)   ( (x) * pi / 180 )
   #define   degree(x)   ( (x) * 180 / pi )
The added parentheses compared to the raw formulas are mandatory to not 
undergo an unwanted precedence change of operators when the macros are used 
in any context of expressions.

