def push(self, from_index, to_index):
        preflow_delta = min(
            self.excesses[from_index],
            self.graph[from_index][to_index] - self.preflow[from_index][to_index],
        )
        self.preflow[from_index][to_index] += preflow_delta
        self.preflow[to_index][from_index] -= preflow_delta
        self.excesses[from_index] -= preflow_delta
        self.excesses[to_index] += preflow_delta