def invalidateChunk(self, cx, cz, layers=None):
        " marks the chunk for regenerating vertex data and display lists "
        if (cx, cz) in self.chunkRenderers:

            self.chunkRenderers[(cx, cz)].invalidate(layers)

            self.invalidChunkQueue.append((cx, cz))  # xxx encapsulate
 marks the chunk for regenerating vertex data and display lists 
renderer.py