sBlock                                       (Assignable)

        Contains an optional code block that, when present, is evaluated each
        time the RadioButto object's state changes.  The code block takes no
        implicit arguments.  Use the RadioButto:buffer instance variable to
        determine whether the radio button is being selected or unselected.
        A value of true (.T.) indicates that it is being selected; otherwise,
        a value of false (.F.) indicates that it is being unselected.

        This code block is included in the RadioButto class to provide a
        method of indicating when a state change event has occurred.  The
        name "sBlock" refers to state block.