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A radio button displays a selection indicator (default shape is diamond) to the
left if its label. Radio buttons are arranged in groups and are used to select
one from a group of mutualy exclusive choices, such as one of the
several fonts or one of several colors.  Associate to each group of radio buttons
there is one internal group variable.  When button 1 is pressed
over a radio button, the group variable is set to the value associated with
the radio button and the selection indicator lights up, and its callbacks
are invoked.
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