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.