Section 11,13: ATAN2

 ATAN2 (real-number, real-number)

 A function that returns the arc tangent of the quotient of the two
 arguments in radians.  If both arguments are zero, the result is
 undefined.  If the first argument is positive, the result is
 positive.  If the first argument is negative, the result is
 negative.  If the first argument is zero, the result is zero.  If
 the second argument is zero, the absolute value of the result is
 pi/2.

 +------+---------+----------+------------+-------------+        
 | Args | Generic | Specific |  Argument  | Result Type |
 +------+---------+----------+------------+-------------+
 |  2   |  ATAN2  | ATAN2    | REAL*4     | REAL*4      |
 |      |         | DATAN2   | REAL*8     | REAL*8      |
 |      |see note | QATAN2   | REAL*16    | REAL*16     |
 +------+---------+----------+------------+-------------+

 NOTE: QATAN2 is not available in DEC Fortran.

 The range of the result is -pi < result <= pi.