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