Description

   The atan2 functions compute the principal value of the arc
   tangent of y/x in the range [-pi,pi] radians. The sign of
   atan2 and atan2f is determined by the sign of y. The value of
   atan2(y,x) is computed as follows, where f is the number of
   fraction bits associated with the data type:

   Value of Input
   Arguments          Angle Returned

   x = 0 or y/x >     pi/2 * (sign y)
   2**(f+1)
   x > 0 and y/x <=   atan(y/x)
   2**(f+1)
   x < 0 and y/x <=   pi * (sign y) + atan(y/x)
   2**(f+1)

   The atand2 functions compute the principal value of the arc
   tangent of y/x in the range [-180,180] degrees. The sign of
   atand2 and atand2f is determined by the sign of y.

   The following are invalid arguments for the atan2 and atand2
   functions:

   Function                  Exceptional Argument

   atan2, atan2f, atan2l     x = y = 0
   atan2, atan2f, atan2l     |x| = |y| = Infinity
   atand2, atand2f, atand2l  x = y = 0
   atand2, atand2f, atand2l  |x| = |y| = Infinity