preBlock                                    (Assignable)

        Contains an optional code block that decides whether editing should
        be permitted.  If present, the Get:preBlock should evaluate to true
        (.T.) if the cursor enters the editing buffer; otherwise, it should
        evaluate to false (.F.).  For Get objects created with the standard
        @...GET...WHEN command, Get:preBlock is initialized with the
        expression specified in the WHEN clause.

        The Get object itself ignores this variable.  It is used by the
        standard READ command to implement the WHEN clause.

        During prevalidation, the Get:preBlock is passed a reference to the
        current Get object as an argument.  Note that this behavior differs
        from CA-Clipper 5.0 where no arguments were passed to Get:preBlock
        when it was run during prevalidation.