A radio-button menu entry behaves much like a radio-button widget.
Radio-button entries are organized in groups of which only one
entry may be selected at a time. Whenever a particular entry
becomes selected it stores a particular value into a particular
global variable (as determined by the :value and
:variable options for the entry). This action
causes any previously-selected entry in the same group
to deselect itself.
Once an entry has become selected, any change to the entry's
associated variable will cause the entry to deselect itself.
Grouping of radio-button entries is determined by their
associated variables: if two entries have the same associated
variable then they are in the same group.
A selector diamond is displayed to the left of the label in each
radio-button entry. If the entry is selected then the diamond's
center is displayed in the color given by the selector option
for the menu;
otherwise the diamond's center is displayed in the background color for
the menu. If a :command option is specified for a radio-button
entry, then its value is evaluated as a Tcl command each time the entry
is invoked; this happens after selecting the entry.