20.4.5 Condition Values
  Many system routines return a condition value that indicates
  success or failure. If a condition value is returned, you should
  check this value after you call a system routine and control
  returns to your program.

  Condition values indicating success always appear first in the
  list of condition values for a particular routine, and success
  codes always have odd values. A success code that is com-
  mon to many system routines is the condition value SS$_
  NORMAL, which indicates that the routine completed nor-
  mally and successfully. You can test for this condition value
  as follows:
  ret_status = SMG$CREATE_PASTEBOARD(pb_id)
  IF (ret_status <> SS$_NORMAL) THEN
      CALL LIB$STOP(ret_status BY VALUE)
  END IF

  Because all success codes have odd values, you can check a
  return status for any success code. For example, you can
  cause execution to continue only if a success code is returned
  by including the following statements in your program:
  ret_status = SMG$CREATE_PASTEBOARD(pb_id)
  IF (ret_status AND 1%) = 0% THEN
      CALL LIB$STOP(ret_status BY VALUE)
  END IF

  In general, you can check for a particular success or failure
  code or you can test the condition value returned against all
  success codes or all failure codes.