ATAN2 (real-number, real-number)

  Class:  Elemental function - Generic

  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.

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

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