ISNAN (real-number)

  Class:  Elemental function - Generic

  Tests whether IEEE REAL*4 (S_floating) and REAL*8 (T_floating)
  numbers are Not-a-Number (NaN) values.  To use this function,
  compiler option /FLOAT=IEEE_FLOAT must be set.

  +------+---------+----------+------------+-------------+
  | Args | Generic | Specific |  Argument  | Result Type |
  +------+---------+----------+------------+-------------+
  |  1   |  ISNAN  |   --     | REAL*4     | LOGICAL*4   |
  |      |         |          | REAL*8     | LOGICAL*4   |
  +------+---------+----------+------------+-------------+