postBlock                                       (Assignable)

        Contains an optional code block that validates a newly entered or
        modified value contained within a given cell..  If present, the
        TBColumn:postBlock should contain an expression that evaluates to
        true (.T.) for a legal value and false (.F.) for an illegal value.

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

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