The correct syntax is:
SET INDEX TO InvBatExp ORDER 1 DESCENDING

>Hi,
>
>I've a problem with the IDX closing automatically :-
>
>INDEX ON Date TO InvBatExp
>SET INDEX TO InvBatExp DESCENDING
>
>Right after the INDEX command, InvBatExp.IDX can been seen disappearing from the Explorer window, on the execution of the SET INDEX command.
>
>This problem does not occur if I use SET ORDER instead.
>
>Well, the problem's solved, but that's puzzling...