The button is shown depressed if the corresponding element of the underlying variable is
1, and raised if the element is 0.
When a user clicks on the button, its state is toggled - it is raised if depressed and
depressed if raised -, and the corresponding element of the underlying variable is
changed. This action can be prevented for all buttons by setting the
`protect attribute to 1, and selectively prevented by
setting `protect to a vector of ones and zeros.
If depressed, the button is shown in `fg, whose
default value is `black (or more precisely, the value of
s.BLACK). If raised, the button is show in
`bg, whose default is `grey (more
precisely, the value of s.GREY), and it is this color
that is used in the shadow of the button, raised or depressed.