def append(self, png, **options):
		"""Append one frame.
		
		:arg PNG png: Append a :class:`PNG` as a frame.
		:arg dict options: The options for :class:`FrameControl`.
		"""
		if not isinstance(png, PNG):
			raise TypeError("Expect an instance of `PNG` but got `{}`".format(png))
		control = FrameControl(**options)
		if control.width is None:
			control.width = png.width
		if control.height is None:
			control.height = png.height
		self.frames.append((png, control))