atan2() computes the angle in the interval [-pi,pi] whose
    arc tangent is y/x radians for x and y in the interval
    [-infinity, +infinity]. The sign of atan2() is the same as the
    sign of y. The atan2(y, x) function 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 > 2**(f+1)             pi/2 * (sign y)
    x > 0 and y/x < or = 2**(f+1)       atan(y/x)
    x < 0 and y/x < or = 2**(f+1)       pi * (sign y) + atan(y/x)

    atand2() computes the angle in the interval [-180,180]
    whose arc tangent is y/x degrees for x and y in the interval
    [-infinity, +infinity]. The sign of atand2() is the same as the
    sign of y.

                           Entry-Point Names

    Generic
    Function   Data Type   OpenVMS           Tru64 UNIX
    Name       Required

    atan2      S_FLOAT     math$atan2_s      atan2f
               T_FLOAT     math$atan2_t      atan2
               X_FLOAT     math$atan2_x      atan2l
               F_FLOAT     math$atan2_f
               G_FLOAT     math$atan2_g

    atand2     S_FLOAT     math$atand2_s     atand2f
               T_FLOAT     math$atand2_t     atand2
               X_FLOAT     math$atand2_x     atand2l
               F_FLOAT     math$atand2_f
               G_FLOAT     math$atand2_g