can-exit? in frame:basic-mixin

Called before on-exit to check whether an exit is allowed. See on-exit for more information.

• (send a-frame:basic-mixin can-exit? boolean

  This, together with on-exit mimics exit:exit.

  First, it calls exit:set-exiting with #t. Then, it calls exit:can-exit?. If it returns #t, so does this method. If it returns #f, this method calls exit:set-exiting with #f.