presto-main/src/main/java/io/prestosql/cost/CostCalculatorUsingExchanges.java/CostCalculatorUsingExchanges/addParallelSiblingsCost
class CostCalculatorUsingExchanges:    private static PlanCostEstimate addParallelSiblingsCost(PlanCostEstimate a, PlanCostEstimate b)
    {
        return new PlanCostEstimate(
                a.getCpuCost() + b.getCpuCost(),
                a.getMaxMemory() + b.getMaxMemory(),
                a.getMaxMemoryWhenOutputting() + b.getMaxMemoryWhenOutputting(),
                a.getNetworkCost() + b.getNetworkCost());
    }