preBlock                                       (Assignable)

        Contains an optional code block that decides whether editing should
        be permitted.  If present, the TBColumn:preBlock should evaluate to
        true (.T.) if the cursor enters the editing buffer; otherwise, it
        should evaluate to false (.F.).

        The TBColumn object itself ignores this variable.  It is typically
        used by the standard READ command.

        During prevalidation, the TBColumn:preBlock is passed a reference to
        the current Get object as an argument.