How to manipulate the rendergraph in Bevy

I would like to be able to get the previously rendered frame and use that as a sampler in the current frame. There is a good set of example code in the Bevy repository to show me how to apply custom shaders as materials to a mesh. But I would like to do something more along the lines of post-processing effects where either the previous frame is used in the current frame or the previous frame is copied to a texture to be used in the current frame.