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A menu-bar% object is created for a particular
 frame% object. A frame can have at most one menu bar;
 an exn:fail:contract exception is raised when a new menu bar is created for a frame that
 already has a menu bar.
constructor
(new menu-bar% [parent parent] [ [demand-callback demand-callback]]) → (is-a?/c menu-bar%) parent : (or/c (is-a?/c frame%) 'root) 
demand-callback : ((is-a?/c menu-bar%) . -> . any) = (lambda (m) (void)) 
Creates a menu bar in the specified frame. The menu bar is initially
 empty. If 'root is supplied as parent, the
 menu bar becomes active only when no other frames are shown. A
 'root parent is allowed only when
 current-eventspace-has-menu-root? returns #t, and
 only if no such menu bar has been created before, otherwise
 an exn:fail:contract exception is raised.
The demand-callback procedure is called by the default on-demand method with the object itself.
Enables or disables the menu bar (i.e., all of its menus).  Each
 menu’s is-enabled? method returns
 #f only if the menu is specifically disabled (in addition to
 the menu bar).
Returns the menu bar’s frame, or returns 'root if the menu
bar is shown when no other frames are shown.
method
(send a-menu-bar is-enabled?) → boolean?
Returns #t if the menu bar is enabled, #f otherwise.
