code: def mouse_area(self, handler, group=0, ident=None):
          
          key = ident or id(handler)
          if key not in self.mouse_proxies[group]:
              self.mouse_proxies[group][key] = MouseProxy(handler, ident)
          return self.mouse_proxies[group][key]
docs: Adds a new MouseProxy for the given group to the 
          EventListener.mouse_proxies dict if it is not in there yet, and returns
          the (new) MouseProxy. In listen() all entries in the current group of
          mouse_proxies are used.