sBlock                                       (Assignable)

        Contains an optional code block that, when present, is evaluated each
        time the CheckBox object's state changes.  The code block takes no
        implicit arguments.  Use the buffer instance variable to determine if
        the check box is being checked or unchecked.  A value of true (.T.)
        indicates that it is being checked; otherwise, a value of false (.F.)
        indicates that it is being unchecked.

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