Before revision:  Our algorithm converges robustly and exactly to the correct backpropagation gradients, requires only a single type of computational unit, utilises only a single backwards phase, and can perform credit assignment on arbitrary computation graphs.
After revision:  Our algorithm converges robustly and exactly to the correct backpropagation gradients, requires only a single type of computational unit, utilises only a single parallel backwards relaxation phase, and can perform credit assignment on arbitrary computation graphs.