def METHOD_NAME(self, path, handler=None):
    """Watch a certain path of elements starting with the DiagramItem. The
    handler is optional and will default to a simple self.request_update().
    Watches should be set in the constructor, so they can be registered
    and unregistered in one shot.
    This interface is fluent(returns self).
    """
    self._watcher.METHOD_NAME(path, handler)
    return self