UNDEFINED

 The UNDEFINED function returns a Boolean  value  that  specifies
 whether the parameter contains a reserved operand.


 Syntax:

    UNDEFINED( x )


 The parameter 'x' must be  a  variable  of  type  REAL,  SINGLE,
 DOUBLE, or QUADRUPLE.  The function returns TRUE if 'x' contains
 a value  that  has  been  reserved  by  the  system  or  machine
 architecture.   If  'x'  does  not contain a reserved value, the
 function returns FALSE.  If 'x' contains a reserved operand  and
 if  you  attempt to use 'x' in arithmetic computations, an error
 occurs.